After a great climb today, I was riding down a long hill & while coasting at high speed (maybe 65-75 km/hr), the bottom portion of my chain suddenly became slack & there was a loud noise that sounded like something rubbing. I stopped immediately & checked to see what may have caused it, but I could see nothing wrong. The same thing happened several times on the 14km downhill from the ski area. Basically every time I'd get up to high speed & coast. I was not touching the shift levers at the time. The final time it caused the chain to come off. It seems very dangerous. I can't figure it out. The bike shifts perfectly. It's a Specialized Allez with 105 rear der. Anyone have an idea of what could cause this?