rear tire came off

[DebW]

Last night before my ride I picked up my bike to turn it around and the back tire came off!

Excuse me for being a stickler on terminology, but apparently you mean your back wheel came off? "tire came off" would imply the tire bead slipping off the rim and the tube exploding. Wheel and tire are not at all the same thing.

The weird thing was that the quick release was still engaged. Is it possible for that to happen or has someone been messing with my bike while it was on my rack? I know I should check that before every ride - but how many of us really do. I probably will from now on though.

I posted about a week ago that when I jumped the railroad track I heard this loud cracking sound - maybe it was the tire coming loose and then going back in place? It's kind of scary to think I may have ridden with the tire being like that for awhile.

The bike was on your car? Was the wheel in its correct place when you picked up the bike? Was the QR tight or loose? Is the QR broken? Does it now appear to work correctly? If a rear wheel loosens while you are riding, it will slide forward until it wedges against the chainstays. It will never slide back where it should be. As long as the bike is on the ground when this happens, the wheel will just jamb in the frame and the consequences won't be too dramatic. If you were airborne, however...