I compared the Leaf regulator to other older Nissans. Many of them had the more common motor assembly that was detached from the regulator track, and that assembly was secured to the door by 3 or 4 bolts. It looks like the 2013 Sentra switched to the design shared by the Leaf, with the motor assembly welded to the track, and secured to the door by one bolt.
Thinking back, I recall that the window in question always had a lot of lateral movement as the glass moved into and out of the fully closed position (more than the other windows, and definitely once I put in the new regulator). And I recall that right before the grinding noises started, there was a very loud, solitary metal snapping and ringing sound. Putting these two things together, it seems possible that for as long as I've owned the car (I'm the original owner), most of the welds were already failed, and the motor assembly was secured by perhaps even just one weld (and bolt) and had a lot of play. Finally that one weld snapped.
Having the motor assembly fixed to the track definitely makes it a little less messy to take out and reinstall, but it was probably a poor decision to have only one bolt securing it to the door. All of the welds on that regulator look like they're in horrible shape. It was just a matter of time before the whole thing fell apart.