Mounting up and riding right off causes this, as does inadvertently toeing horse in sides, coming down too heavily and dragging foot over rump.
But it can also be that you never make horse stand still, such as when haltering. If you put halter on, and immediately move off, you are teaching him to not wait for your cues. I halter, make them step back and then we stand. I also make them stand periodically when leading, as well as speeding up and slowing down, so that horse understands I will decide when to move.
Making your horse stand stock still when riding also helps to reinforce that you are in charge too.
Horses make me a better person.