The crumbling is caused by the muscle fibers tightening up to the point where they've become brittle, which can happen when they've been exposed to too much heat and/or acid. I'm guessing that your tenderloin is either being overcooked, over-marinated, or both.
Tenderloin differs from tough cuts like pork shoulder, because they don't contain a much collagen which provides structure when less cooked (think: a tough cheap piece of steak,) or turn into rich gelatin when cooked for a long time (think pulled pork or pot roast.) Since tenderloins have little connective tissue, it will be nice and tender if the muscle fibers are treated gently, (think: medium-rare steak) but won't have all of that nice collagen and connective tissue in there to mitigate over-marination or overcooking (think: dry, crumbly meat.)
- Switch to a flavorful wet or dry brine, or cut down on the acid in
your marinade and only marinate them for a couple of hours.
- For doneness, I prefer my tenderloins cooked to about 140F, but
you should be able to get closer to well-done without them falling apart, especially if they're brined. Once you start getting into the 165F+ zone, you're going to be looking at a pretty dry piece of meat.