It's an evolutionary precaution! Sudden movements initiate what's called the "fight-or-flight" response. Your heart and breathing rate increases, adrenaline levels rise, and so on. The "jumping" or lifting of your hands is a reflex to protect you, either to move out of the way of something that might be flying towards you (thousands of years ago, a predator, or a branch falling, etc), or to protect yourself from being hit. Most animals have this response - you might have experienced this if you have a pet dog or cat, and drop something on the floor.
Even if you might be "expecting" it, as with the toast popping up, you're still not sure exactly when it's going to happen, thus your body responds in this involuntary way. I might do a hub about this soon; it's an interesting topic.