Broken clutch backing plates inside 6t40 transmission

Excessive internal pressures, doing "neutral drops" (IE, rev the engine in neutral and yank the transmission into gear) will both cause broken plates. Normal operations won't usually break them, but abuse does frequently. That unit isn't known for its strength under strain.

Another item that could cause it but isn't very common would be if the clutch apply piston was clamping at an angle, but that would have done more damage than just the plates. The only time I've seen it happen was when one of the steels between the clutches was damaged, however that also would cause slippage and burns up the clutches very fast, the transmission wouldn't have made it 50,000. Those are just a part that doesn't normally cause problems.