I have had this happen a couple of time, mostly on smaller butts, and when you leave it on, when they do kick over and start getting tender it goes super fast. I can't explain it, I assume it has to do with the nature of that particular cut of pork. Although, it has definitely been with smaller butts for me.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."