I have had the same problem before and I found a really great tip for this.
As the rider is mounting they need to watch if the horse is going to move. If the horse does move, stop it and back it up a step. Then try again. It make take a few tries but this definitely worked for the same problem that I had.
Download the worksheet to test your knowledge! How much hay and grain does your horse get every day? How much SHOULD your horse be getting each day? Many horses may not be getting what your feed bag recommends,...