Complex sentence: It turned out that the man sent out by Agnew had fell from his horse when being fired at by the Jacobites, as the horse was so scared but he had escaped on foot and was able to make contact with Crawford.
Split into simpler sentences: It turned out that the man sent out by Agnew had fallen from his horse when being fired at by the Jacobites. The messenger escaped on foot and was able to make contact with Crawford.