Strange thing happened ...

I put on a record this morning and felt something was off and later realized the soundstage shifted left (between center and left speaker). Putting my ears against the speakers, I could hear sound from both but couldn't make out the difference in levels. Moving back to my listening position, soundstage was still tilted left. I changed the source (Streaming) and the soundstage was back to normal.

Back to TT source, I swapped the input cables at the amp, the image moves right. Swapped the cables between Phono and SUT, image shifted to the right. Swapped the cables from TT to SUT the image again shifted right. Now it was conclusive that it was the TT.

Now I pulled the cart.leads and I felt the right channel +ve clip was loose. I crimped the clip a little and pushed it back on to the cart. Pin.
Put on a record and voila everything snapped back into the center.

Not sure what caused the clips to loosen. Just wanted to share my experience.
This happened to me. Turned out to be a broken solder connection in one of the XLR between pre and amp. Had it fixed and all's well.
When that sort of stuff happens to me (and it does happen on occasion), I check all the connections for that channel.  I'll rapidly twist the volume control.  I'll rapidly twist my integrated's input selector. I'll lift the tubes up and down in their sockets. Nine times out of ten, one of these things'll cure the problem.  More often than not it'll make the system sound better, as well.  Sometimes connections just get grungy. 
1+ @noromance 
If you change cartridges enough this is bound to happen. Most good tonearms have very excellent wire that is capable of taking a lot of abuse.
It is always the junction at the clips that fails. Once one fails the others are usually not far behind. Get a good set of clips that come with colored heat shrink like these
The heat shrink is critical as not only does it allow you to identify the wires but it serves as a strain relief moving the stress away from the solder joint. If you don't do this the wire will break at the solder joint every time from work hardening. The clip has to be held firmly. If you do not have a little vise get some modeler's clay and just push the clip into the clay with the solder end hanging out. If you do not immobilize the clip doing this will be a frustrating experience. Don't forget to push the heat shrink up the wire before you get started!
@edcyn I do those things sometimes except that pulling and reseating the tubes is not easy on my integrated. For that I'll have to pull it out of the rack and remove the top cover.

@noromance It was a logical bottom-up approach

@mijostyn Only on my second cart on this table so not too many clip pulls and it is a Rega arm so not the best clips out there.
It is crazy how the smallest detail or problem can cause something like that i had the same problem with a bad speaker wire. The great thing is that you found the problem and could fix it so easily.
@aewarren NP@speakermaster it surprised me as well that a loose cartridge clip can cause this issue especially since both channels were working just that the right channel was low on level. Actually before I got to the leads I first confirmed cartridge alignment was spot on.
I have had the same problem in the past, too.   Those little clips loose tension and sooner or later you will need to tighten them.