Curious if I can get a sanity check off my problem diagnosis (or alternate theories!)

I tried a long print today and wound up with a 1/2 inch layer shift on the x-axis near the end of a long print, taller than most I’ve done, not certainly not the tallest.

It occured on a spool I just opened a few days ago and printed two other ~250g pieces with. I’m very certain that I never lost control of the filament end. My spool in mounted using the stock ender 3 mount on the left side is the gantry and a filament guide arm.

After reading a bit, I’m thinking this was due to the filament on the spool loosening up from a large travel and then binding on itself. Seems the easiest way to fix this might just be to put more space between the spool and the printer so the slack can absorb the shifting without pushing back on the spool and loosening several turns off filament.

I don’t think it’s heat or any general axis binding as the shift only happened at a single layer, at a hight that I’ve been able to print through before, and the motion generally appears smooth when I exercise it.

So… Experimentation will probably prove me right or wrong, but before I sink another day of print time… Does that sound reasonable or am I missing a common problem?

Edit: Solved, see comment by @[email protected] for the actual problem. Many thanks to all who provided their thoughts!