So far since late spring I have had four problems on my mountain bike all relating to the rear mech area.
1) A spring broke on my old mech that couldn't be user-replaced.
2) Soon after fitting a new mech a large twig wedged things and broke the hanger.
3) About a month after fitting a new hanger the mech unscrewed itself and damaged the cable and chain (I guess this was my fault but I tightened it as much as I thought reasonable at the time).
4) Today the new hanger broke. No twig or anything this time. The chain skipped a bit, appeared to re-settle then everything ground to a halt with a crunch. I was riding slightly uphill on a bridleway in a field at the time and there was nothing to catch in the mech. The jockey wheels were a bit choked with mud from an earlier section but on inspection it all fell away very easily. My idea of how it broke is that when the chain skipped it somehow got stuck in the mech around the jockey wheels and my pedalling then dragged the mech round until the hanger broke.
My worry is that there is an underlying issue that I have not found and it may happen again. Is this likely? Any suggestions? (The chainline looks OK and the shifting was fine up to the breakage).