Kicking back, bucking, - all evasions trying to tell you something and this horse is shouting that his stifles need veterinary investigation and they are locking.
It is not surprising that the horse won't load into the trailer with the partitions - it is way too small and low for that horse and he knows it. He won't be able to balance himself and has probably hit his head on the top before.
@Maanlingxx Just look at the horse.