I'm trying to figure out what happened today with my bike. I was backing it out of the racks at work, and I thought I heard a noise when backing it up (will come to that later), but started talking with a co walker and completely forgot about the noise , and went to start riding, and as soon as I make a complete turn of the crank, a horrible grinding noise starts, and the crank locks up. I look down to see the chain has literally worked itself into a knot pretty much, and has fallen off the crank, and is all sorts of jammed up in the rear derailleur. The derailleur is jammed so far up its at the back of its range. I broke the chain and assessed the damage, and I had to force the RD from its stuck position, but the spring is working now, and it doesnt appear to be damaged.

I should note the bike is an 84 cannondale, so there is no RD hanger, just a bolt mount.

Why would the chain have slipped off when coasting backwards? I was in the 2nd to highest cog, so I can't imagine that the RD came in contact with the spokes, as I had just climbed the parking lot hill in the largest cog right before that. The chain was pretty old, and maybe that was to blame? I bought a new chain now but am hesitant to put it on fearing that the same thing might happen again