Detuning. Yes, they break. Right where the string enters the post and it locks, it usually ends up facing away from the nut and bridge once it's locked and tuned up. It creates a pretty sharp angle in the string here, and that's where it breaks if I detune, and then tune up again. Not always, but not infrequently either.
It's been the same with PW locking tuners, locking Grovers, and locking Gotohs as far I remember.