Rear derailleur got caught in the spokes, what could be a root cause I have several problems on this bike. Here are the elements that have failed and other pieces of information:


the chain has derailed past the largest sprocket
the rear derailleur has rotated 180 degrees, and is caught in a spoke (between the frame of the derailleur and one of its sprockets)
half of the derailleur is now bent 45 degrees from its frame (derailleur on picture 2 should be U-shaped, not V-shaped)
sprockets showed some signs of wear, but not to the point where it causes the chain to skip when load is applied
the threaded hole to which the rear derailleur is screwed shows some sign of wear ( this is a modular part which is itself screwed to the frame)
rear axle is broken in half right in the middle. I don't have the tools right now to open it, I believe this requires a punch and a hammer to remove the cog set.. Right now I have already lost some bearing balls after I moved the axle from the dropouts.  


Now I'm wondering if there is a single failure that could explain what happened. 

Thanks to the answers, I inspected the derailleur closely and found this:

There is a missing chip exactly where the L screw is supposed to hit the frame. Apart from that and the fact that it's bent beyond repair (IMO) the derailleur is actually brand new..

Here is a picture of the broken axle I  knew something were really wrong when bearing balls started falling on the ground...
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