Originally posted by Kzar:

They switch targets AFTER they actually see them.

They were hitting on my boars even as I was hitting on them, so.... yeah.

[EDIT] And this isn't a one-off thing, either. It's happened on multiple recent playthroughs.

[EDIT 2] Also, it's happened on more than one occasion, when I had an earthen wall for defense, they would stand there looking towards the boars while I was OUTSIDE the base, plucking them with arrows or walloping them with a mace.