Can anyone explain this?

I’ve had a Rega turntable and Grado cartridge combination for many years. Both are known to cause hum.  I’ve lived with this hum, thinking there was no way to alleviate it.
  A friend suggested I get a transformer that would eliminate it.
So, I ordered a Jensen ISO-Max transformer which arrived today.
Hooked it up.  No better.  The hum remained.
I removed it and plugged the cables into the preamp as usual.
THE HUM DISSAPEARED! at normal listening levels.
Now, through the years I’ve hooked up and unhooked the turntable several times in different positions.  Always the same hum.
Can anyone explain this phenomenon?
7c67ab18 c2ce 4b45 9523 fc4a71684ce0rvpiano
You are LUCKY!  You solved your problem easily.  Some have glitches in their system that they work on for many years with no solution.
Now don't touch it :-)

As boxer12 said, walk away, don't taunt the audio gods, just walk away.
The audio gods work in mysterious ways.
Could be a weak solder joint in your cables or cables touching other cables creating "an antenna" and during the swap out you happened to get them oriented just right. The slightest bump of your connection points and it might reappear. Just an educated guess, but what else could it be? Cheers,