The mystery of the broken bike wheel!

I, being the wonderful and adoring wife that I am, went out of my way to buy my husband a bike last week. Of course I bought one used because I'm a cheapskate. I'm not a bike expert, so when I bought it I just kind of rode around, changed gears used the brakes, etc. After I brought it home I tried it out on a longer ride, heading up to the top of the levee by my house. I liked it. It was a smooth, fast ride and the bike was light-weight for a mountain bike. I put it away in the back yard in anticipation of a morning ride with the hubby.
Here's where it gets weird. The next morning when my husband got on the bike he tried to peddle it and it wouldn't go. The back wheel was stuck - actually it was hitting the forks, completely bent out of shape! Somehow the wheel had become so severely bent that it couldn't even turn - yet the evening before I rode it all over with no problem. Surely I would have noticed if I hit a bump so hard it mangled the wheel. But what could possibly have happened? It is a mystery! Frankly I think my neighbors borrowed it. They're kind of shady and often eye my bikes with jealousy.
That's my theory. David's theory is that the wheel was bent before and the guy who sold me the bike bent it back, but David's heavier weight on the bike popped it back to its bent-ness. Hmmm.
What's your theory? If you figure the mystery of the bike out, you win a bent bike wheel! How cool is that! SO put your Nancy Drew caps on and help me with this one!