The rider riding backwards (too much hand and/or not enough leg) when the horse is "having a look" at the fence can easily cause a refusal.
"When training, riders should focus on the line, but sit quietly and allow the horse to use its own initiative to jump the fence." ~ Eric Smiley
It is tempting to want to repeat something over and over when you feel like you are making some progress on a movement you have been struggling with. But it will be more productive to reward one good effort, give the horse a break, and move on to something else
In the sport of Eventing, ensuring that our horses are truly fit for our level of competition is one of the best ways to prevent unnecessary injuries.