I have heard of a number of possible causes for this. I have one gun that had this issue and with that gun the hammers don't rebound until the gun cocks. That requires the gun to begin to open but it can't open without a considerable amount of effort if the firing pins are still embedded in the primers. When the gun is opened you can see where the firing pins dragged leaving elongated dimples on the primers. That is a good way to break a firing pin. Some people have corrected similar issues by rounding, shortening, or otherwise modifying the firing pins. I was using Winchester primers but I started only using Fiocchi primers in that gun and haven't had a problem since.
