Answer

By Your Horse

Natural Horsemanship Expert Charles Wilson says:

For some mares, being protective of their rear end is an instinctive reaction. She is being defensive of her own space, rather than aggressive to others. It can be a hard habit to break but, if we work to make her feel more secure, you may see improvements.
Ride her forward and keep her attention on you. For instance, if you’re out hacking and you feel her tense up, ask for a slightly faster pace, giving with the reins and urging her forward – she can’t move forward and kick at the same time. Don’t punish her if she does kick, just find and keep the forward momentum.
There is an exercise that may help to desensitise your horse, but you’ll need the cooperation of a few friends with steady horses.
You are the centre of a carousel – to begin with you can try this dismounted. You and your mare walk a tiny circle. As you are walking, two mounted friends will begin to circle about four metres around you, or even further if she becomes bothered by their presence.
If all is fine, ask your friends to trot on their circle. As time goes on, your friends can slowly shrink the size of their circle around you and your horse – but make sure you keep your horse walking whenever the horses come in closer. The next stage is for you to do this mounted, remembering to keep her moving until you feel she is completely fine about the horses going past. A few sessions of this will accustom her
to horses being near to her.
Always reward her when she needs reassuring, or has tried hard.