Wolves. They're buggy at best.
One of the packs on my Medved go aggressive when I don't know where they are and by the time they reach me they are full-on sprinting. Constantly.
It happened once - then again. Now I call them Russian wolves because someone has obviously given them a drink.
Haven't seen that pack do it for a few days - haven't found them either. Another pack on another part of the map just did it though. I thought it might have been due to the distance of them going aggressive - the first pack did it soon after I fast travelled to a tent. The most recent pack did it after I shot at one.
See videos. Ignore my godawful aim. I'm great at this game.
https://www.youtube.com/watch?v=QVmsISylgAA
https://www.youtube.com/watch?v=TGHZbruccRw