Horse attack on a dog

My dog got attacked by horses!!
Hubby and I were walking along the fields following our usual trail along the public foot paths with our dog Trudy. We went over a stile into a field with horses. They are usually very friendly and I alway have a couple of them coming to me for a little "scratch".
Last Sunday, things went weird.
Trudy is 16. and arthritic. She cannot run like she used to and she usually stays near us when walking. Somehow she got separated from us being scared away by one of the horses and ended up on the other side of the field, behind the fencing. The horse would just wait for her on the other side and every time she tried to come back to us the horse would go after her again.
Eventually, the horse walked away so Trudy tried to cross the field. The horse galopped towards her and caught up with her very quickly. Trudy could not run fast because of her age.
Anyway, the horse got to her level and hoofed the top of her bottom with its left foot.  Trudy's back legs gave away and she yelped.
I thought the horse had just broken my dog's pelvic bones. Here she was in the middle of the field not moving with her bum still on the ground. As all the other horses starting to circle her, I just went under the wooden fence and ran towards her moving my arms in the air trying to get the horses to move away and shouting at them. I picked my dog up and carried her away. Then, hubby said. "Hurry up the horses are following you." So, I did a couple of  back-kicks hoping this would make them keep some distance. Apparently, that startled one of the horses and it moved away.
Once safe, I put the dog down and luckily she managed to walk. She got really stiff the next day so we gave her some Ibuprofene with her breakfast the ease the pain away.
She had a big lump where the horse hit her for a few days but she was back to normal after a couple of days.
We have not gone back to the field yet. I think next time you do, we will have to put her on a lead to keep her safe.
I am quite confortable with horses as I used to ride them when I was young. I have never seen such a behaviour before. Trudy never bothered them. She never ran after them or even barked at them. This was an unprovoked attack.
All I can think of is that they must have had a bad encounter with another dog...