After they've been told to follow.
Anyway, I tried what you said, and yes, they start firing without the PC attacking. I don't really mind, if I change their perception range to short, there won't be much trouble.
The NPCs have managed to fight a little while longer, but still stop at some point during the attack. Weird. But maybe that was because this time, I wasn't fighting. I thought that because the NPCs began fighting without the PC fighting, it didn't matter anymore.