Complex sentence: The momentum of the battle turned when a musket ball hit Macdonell's mount (causing it to rear and twist around) and another shot hit him in the small of the back, causing him to fall from the horse.
Split into simpler sentences: The momentum of the battle turned when a musket ball hit Macdonell's mount, causing it to rear and twist around. Another shot hit Macdonnell in the small of the back, causing him to fall from the horse.