If you're doing your burnouts starting in first gear, this could be the culprit. When the tires are loose and the TH400 is shifted into second gear the sprag gets subjected to a pretty high shock load which can cause it (the sprag) to fail. An easy fix for this problem is to always start your burnout in second gear.
Get your trans out and disassembled, the sprag is easy to replace and you don't want to take a chance of jamming any of its schrapnel into the case or drums.