Hitting an enemy from behind with a "ranged" weapon allows them to spin 180 and fire back. I think wiki has the article on whether an enemy gets a reaction fire after a certain action, you can read more about every situation. There are ways to circumvent it, use a regular melee weapon from behind, or attack first with a weapon that has reaction disrupt property. Stun grenade, spear and leather whip come to mind. They even scale from the same stats as a pike.