Avoid crumbling pork?
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.