Another possibility is that she is concerned
about what is behind her. Doing what you are asking of her, when she
can't see what's behind her (such as another rider on his/her horse), takes
her defense mechanism away from her.
Possibility, but
highly doubtful with this particular mare. While many OTT STBs make good
riding horses, she was one that hated the idea of being ridden. Wanted
to be a pleasure driving horse. Her wish was
granted.
