Why does a person fall off the back of a stationary horse if it darts off suddenly?

I have experienced this myself so I have a say, well when the horse darts forward, maybe cause you kicked or whipped him to hard or he got spooked, there is more reasons but the one when the horse gets spooked one is the most relevant, anyway you fall off because the horse can do it so intently that you can take it by surprise and fall off because of the sudden change of speed.