That's just it, we can't tell what the horse's motivation is based on the OP. He could just be bored and looking for excuses, the rider may not be doing a good job keeping the horse's attention centered on his job. They horse may just be so genuinely afraid of the whole situation he has to get out of there.

Regardless, the OP has allowed this to become enough of a habit that she's going to have to get outside help to break it, preferably someone who can stick with anything the horse throws at him/her and work the horse through it.

The OP will need to be extremely consistent about keeping the horse's attention on her. She can be soothing and praising, that's fine, but she'll need to be absolutely the last authority to the horse.

Whatever she does, she needs to not be taking this horse out by herself and without a trainer until it's resolved, she'll be only making matters worse and reinforcing the behavior with the horse.