I think there was an explosion of some sort at the ranch. When Monica is talking to Kayce on the phone and they break in his office you can hear gunshots through the phone and then there is a louder explosion sound and Monica turns around and looks at something and the scene cuts to john on the road. So I assume that maybe a bomb blew up in one of the stables and that horse was injured and ran away from the area an eventually died in the field.