I too would want to completely rule out saddle & other pain/discomfort. As you're moving your leg back, you will be rolling your weight more forward on your pelvis. If the saddle isn't great, this movement will make it worse.
Other than that, sounds like your horses need more desensitisation to things like that, and to be taught to yield(respond softly with understanding) rather than react from fear or confusion. I'd be doing this from the ground first, as I think it's more effective, as well as safer.