The other week I brang my Mountain Bike to a bike trail, The trail has up hills and jumps. When I was going up hill I down shifted to the lowest gear while I was pedaling hard and suddenly my rear derailleur hanger had broken. I didnt know how this happened
chain probably shifted past the larges cog in the back and got caught between the gear and the spokes. It's usually all over from there. That would a misadjusted derailleur if you're looking for a culprit.