It's a known AI issue. The AI seems to think the PC is the bigger threat in most situations and goes after them first. It's definitely on our list of combat issues to fix.
This was glaringly obvious in the "survivor encampment" (I'm not sure if I got the name right)
I found a location on the map with a survivor camp under attack by zombies.
The zombies attacked the survivors, killing one before I even acted.
Being a good person, I went to try and kill the zombies to help the survivors.
The survivors then immediately changed their target to my team even though they were still being actively attacked by the zombies.
Hopefully you will fix this soon!