When I rode motorcycles, I learned an expression - there are two types of riders: those that have been down, and those that will go down. You can't always predict when or how it's going to happen. I was riding on the highway when a mattress flew off the roof of the car in front of me (twine just wasn't strong enough at 70 MPH, I guess) and I had vehicles on either side of me. I just held on and rode straight over it and kept going. Another time I was making a very low speed turn, hit a small patch of sand, and found myself laying halfway under the bike.
I'm sure there are a lot of physics involved in a fall. Things like speed, lean angle, etc. Usually you won't have time to figure out all those computations in your head when you realize you've hit something in your path, and you're just along for the ride. The best you can do is to try not to hit things, I guess.