Horse accident caused by dog?

Neighbors of ours were riding the road in front of our house on horses. My dog ran across the yard barking and the horses spooked. The lady fell off and had to be lifeflighted. She only had a broken collarbone. The dog never left our yard and the rider fell on the township road. We don't have the dog fenced in but like uli said he nnever left the yard or jumped/ bit at the horses. Who is responsible? Isn't it an assumed risk that horses are dangerous and u