motor mount not tightened enough or evenly?
maybe screws that hold the spur loosened?
motor moved in a crash?
too tight or loose of a mesh to begin with?
My T4 has seen a few thousand laps so far using the same 64pitch spur that I put in when I built the car. I am only running a T4 in 17.5 blinky right now, but if there was an inherent design flaw causing spur gears to strip, I figure I would have noticed it. She's taken a few good hits including one that sheared off a ball stud.