After dealing with a group of bandits manually, the NPC hirelings I had involved in the battle turned on me.  I didn't cast an AoE that hit them, and I don't think the bandits cast any spells that would have turned 3 on me at once.

It's possible an enemy turned one of your hirelings and one of your party members hit them. Something similar happened the first time I defended the stronghold manually (after reloading because I used Restore Light Endurance on one of them to keep them from dying, which is interpreted as an attack. LOGIC). The attackers turned one of the hirelings and Aloth hit him with a blast after the enemy had been wiped out, which caused all the hirelings to turn hostile.