of three reasons: 1 - You hit water, or gravel, or wet leaves, or something else slippery while in a turn. 2 - Someone else did not see you. 3 - You were doing something stupid. Don't take hard leaning turns unless you are absolutely sure of the terrain. Drive like you are invisible and assume every other vehicle on the road is going to change lanes or pull out in front of you and have your escape route planned in advance. Oh, and don't do stupid shrit. Stick to those three rules and you should be able to avoid most wrecks.