Lou, I do not know the specific circumstances that caused the failure of the ER, but I can say with 50 years experience shooting S&W's, it is a very rare occurrence. To happen to you twice and both on 8 shot revolvers puzzles me. I wonder if they are more susceptible to breakage than the standard 6 shot revolvers.

I must ask.... has the ER been removed and reinstalled prior to this happening? Maybe it was tightened a bit to much. Could it have been turned the wrong way during take-down?

Without more info, I can not do much more than to guess.