Henchmen sometimes turn hostile when caught in the PC-caused area-of-effect damage (IMO, the Robe of Balpheron may as well be called the Robe of Henchmen Rebellion). Usually, killing them and resurrecting will reset their reactions. There are also scripted solutions. BTW, you can use the console to reset your XP so that you don't get credit for killing a henchman.
I pretended that that Deekin was manipulated by some spell and took his death XP gladly! I wish i could kill him as much as i had resurrection options.