The deer have mistaken themselves for cows.

[KoE]

I've had this happen in the areas forests Bottleneck, and I'm not sure exactly what triggers it.

Usually, when I encounter more than one deer, I'll queue up all of them into combat with my fisticuffs. Even with pathfinding, this causes something like a train wreck (only with more deer than average) that keeps them from pathing to me. And, strangely, it seems any deer that doesn't have a straight path (I'm not sure if this is the trigger or not, but it happened when one collided with a rock too) reverts to the not rabid-and-gonna-stomp-that-jerk-with-the-bow state. And, in fact, just mill around like cows until you kill them. They don't even try to run.

I can only imagine this is not intended, though I do appreciate the easy deer dogs.