UKH

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).

Have you replaced the cassette or chain recently? If the mech was old enough to break randomly, I'd worry about the rest of the drive train. If worn, a slipping chain could jam and the pressure of pedalling uphill can easily do quite a bit of damage. 

All less than a year old except the small chain ring which is in good nick.

 When you say the hanger broke, did it properly break in two or just bend? And I'm assuming that if everything is newish, it doesn't usually skip. It's it possible that something got caught again? 

Bent links in chain?

Bent teeth on any cogs?

Sometimes these things just keep happening...I've had 6punctures (tubeless tyres) in the last 2months, having had 1 in the previous 12months. Hoping it's just a spell before normal service is resumed!!

It broke in two. I tried to explain that there was nothing around at the time of the 4th  incident that could have got caught except some easily dislodged mud around the jockey wheels.

New chain after 3rd incident, cogs all look  decent. I too hope its just bad luck but wanted to try and make sure I haven't missed something.

1-3 sounds like bad luck. 
 
Point 4 sounds like it may be a bit of a set up issue. Could be a number of things but chain length and ‘b’ screw set up may be things to look at. 

After the earlier incidents - is the hanger actually sitting straight or has its mounting point been pushed out of alignment?

Get the frame / gear hanger alignment checked - I've straightened quite a few frames where the hanger has snapped and distorted the mounting on the frame.  

The usual reason for failure for 4) is that the top jockey wheel comes into contact with the large sprocket or the jockey wheel cage hits the spokes.

Thanks, I'll be careful about those when the new hanger arrives

The frame looked OK and the new hanger sat flush in the recess. I did a couple of short test rides carefully at first  then with less concern and there didn't seem to be issues with the mech hitting the spokes. On the morning of the last incident it was hard to force the chain on to the largest cog so I backed off the limit screw a little. Maybe too far? It seemed OK but I suppose if it was just the cable holding it away and it bedded in just a little more??