This is hypothetical in the spirit of a recent question about derailleur cables, but I've seen two bikes being pushed with broken rear derailleurs in the last few days.

What are likely causes of broken rear derailleurs (commuting /road especially; I assume that in MTB it's hitting stuff). Alternatively what can we do to avoid it? I've worn one out but never broken one.

Perhaps more importantly, if a rear derailleur breaks on the road, what can a rider do, and what tools would they need?