Rear derailleur got caught in the spokes, what could be a root cause
If the low limiter screw on the rear derailleur was very badly adjusted, it would be possible to move the derailleur far too far past the end of the cassette by shifting into the lowest gear (i.e. shifting onto the largest cog on the cassette). This would move the chain off the top of the biggest cog and could force the derailleur into the spokes which, in turn, would bend it.

There's a lot of information on setting derailleur limiter screws on the internet - here's a fairly good example