why did my record player slow down, speed up, then go back to normal?

why did my record player slow down, speed up, then go back to normal? it all happened within maybe 12 seconds. it slowed down, then sped up immensely, then went back to normal speed. why?
Space aliens. Without more details it is hard to say from what planet.
Well the flux capacitor has to charge up fella..

Cap charge, my guess. Lose power connection.

Every time or just once in a while or JUST one time?

Too little info, so all would be a guess. On start up, while playing, how often, type of drive....
First, is it idler drive, belt drive or direct drive? If idler, perhaps the idler wheel needs resurfacing, if belt the belt could be slipping. You can put the belt in a bag with talcum powder and shake it up and reuse the belt. If it still slips, get a new belt from the manufacturer.  If it’s direct drive, the speed controller circuitry could be defective.
Brief power outage or surge, bad capacitor, belt slippage, bad bearing, severe stylus drag, finger on the platter, platter rubbing on plinth, Schoenberg, bad karma for having been a troll.
If this is an older turntable with variable speed, one explanation might be that the speed controls or the speed selector switch are developing corrosion. If this is so, you can expect future events and more often.

Another explanation is a brief power outage.
On a deck with a synchronous motor (Elite Rock) a new phasing capacitor resulted in the pulley just quivering back and forth, until the deck was turned by hand when it kept going. After a while it would start on its own in the right direction but with momentary misses, which went after a few days. Was the new capacitor forming up after being unused for too long? No idea but the speed glitches never came back in ten years.

On a deck with a brushed DC motor and a feed forward power supply (Artemis SA-1) it was a symptom of the sintered motor bearing running out of oil. Getting some more in required removing the pulley but the adhesive had gone soft so it could be eased off and 100μl of oil (I used Nye Lubricants 181B but the designer recommended liquid bearing, however they wouldn’t supply that outside the US) run down the shaft to soak into the bearing, I did this 25μl at a time rather than all at once. When I replaced the bearing with a bit of loctite 648 retainer to hold it in place I made sure there was enough gap to get a micro syringe needle into to top up the oil in future.

There isn’t one answer to this one.
I wish i knew the table but if it was a direct drive maybe the drive servo had to reset itself.
If it is a table with no fixed speed controller, it could have been a surge or power change in the system temporarily.  in other words, if a large load came on-line in your area, the frequency would changes for a very short time so that additional generation can catch up with the load.

So, if you have a 60 hz system, it is fluctuating between say 59.5 hz and 60.7 hz.  (just an example).  So, most people use a stroboscope disc and a regular incandescent bulb and adjust the turntable speed for exactly the correct speed.  But, this only works if the system frequency does not change and it always changes.  Electrical system controllers and auto transformers adjust voltages to compensate for  system load changes.

So, you could have experienced a system surge in your area for a short moment.

If you have a speed controller, that is different.   A speed controller takes the 60 Hz frequency (USA) and voltage and converts it to DC, the converts it back to a fixed line voltage and a fixed unchanging 60 Hz, making your turntable speed always fixed regardless of changes in system frequency from the wall.

Unless one has a speed controller, internal or external, the turntable speed is subject to changes based on the electrical system changes based on loading.

Of course, this is assuming that nothing went wrong with your system.

I hope this helps.  

For fun, get an inexpensive (a few dollars) stroboscope disc and a incandescent bulb and check the turntable speed.  

There are also apps on line that provide for your cell smart phone to do this same check.

It is interesting.

Apps such  as RPM, Rotor or true note.

Happened to me with my older Luxman PD-277.  There are speed control potentiometers for 45 and 33 RPM and they were showing their age.  A little DeOxit cleaned them up.  The wiper contact was a little dirty.
atmosphere is correct. The speed potentiometer might be corroded. This happens a lot on old technics 1200 tt. It's also could be the speed control module. 
If it’s a belt drive, the belt may be stretched or worn. Could be a dirty speed control potentiometer! If it’s direct drive, could be a problem in the servo drive ckt. 
Good lord. Love to see the usual suspects as the first ones chiming in with snark and nothing to add. 
Now after a full day of denial the military has released evidence of multiple sightings by pilots and radar. What was originally denied to even be happening is now openly discussed as being due to space aliens. Unfortunately too late, as we can see from the lack of replies the OP has been abducted. If only he had known to unleash the yodeling. https://youtu.be/aTe0MjAZvMU?t=49
Good lord. Love to see the usual suspects as the first ones chiming in with snark and nothing to add.

Like you just did???...........
I am a vast well of helpful information. For instance, you may wanna get your keyboard checked out. Judging by your previous posts, the period key seems to be damaged or get stuck. 
Lee, has this only happened once or does it do this intermittently? If it happens occasionally I second the speed pot degradation theory. Good starting point to check at least 
Depending on time of day and prevailing weather conditions driving load against the power grid, it might have been a fluctuation in available current in the power grid feeding electric service to your home. It happens on a weekly basis for our power grid. The various fans that help our ancient HVAC system move air in our home will slow then speed up at various times through the day, much too subtle to cause the lights to dim. Once the load stabilizes after 10pm or so all things plugged in are much happier. 
hey guys. thanks for all the responses! it is a belt drive, and i was about 25 minutes into one side of a record (CSNY's 4 way street, side 3, southern man, in case anyone wants to know). It was kind of a humid day. This has only happened once.

thanks again everyone. any input helps.
@stereo5 thanks for the tip! i'm going to do that, even if the belt isn't the problem. (it probably is, but then again, it only happened once for about 30 seconds.)
@atmasphere thanks. this is an older turntable, but it has electronic speed buttons (33, 45 rpm). I think i'll check the buttons for corrosion.
i guess it could have been a power surge from what everyone is saying. It might've been the belt, but it had never happened before or since. 

man you are funny! you have kept on talking about aliens throughout the post. i don't care if you keep going, i like having you around.
Didn’t spend enough on your cables and contact cleaner; )
is it belt drive? Did the belt ride down on a crowned pulley possibly?
If the table has a speed control dial it most likely needs to be cleaned with contact cleaner.  These are sometimes located underneath the table or some have controls on both the top and bottom that each need to be cleaned to keep consistent speed.
I actually went to play a record today, and I had to press the (electronic) buttons multiple times before it did its function. There was no speed swell, though.

I am thinking that the buttons are corroded. 
Also, @stereo5 I did the talcum powder thing today. Thanks again for the tip! 
I actually went to play a record today, and I had to press the (electronic) buttons multiple times before it did its function. There was no speed swell, though.

I am thinking that the buttons are corroded.
I put myself through engineering school fixing problems like this. Its very common on older turntables. The speed selector switch can have corrosion, as well as whatever controls might exist for setting the speed on both 33 and 45.