These are standard heat treated spring steel internal circlips. If you have tried to destroy one, (I have), you would know they are virtually indestructible. These circlips are not designed to have force exerted on them and in this situation there would be very little. I am also extremely doubtful that they would fail due to vibration. It would be more plausible to think that the groove in the piston would be the problem, but even then it doesn't make sense to me as there would be minimal pressure applied to the circlip.

Obviously there is a problem somewhere and it would be very interesting to know exactly what it is. It's more than likely we'll never know the real issue.