Though the stone wall of the outpost is very sturdy, yaks keep charging it and creating pits in the exterior. It's probably because it's mating season.
If they keep striking the wall, they'll damage it beyond repair. You should be able to stop them, right?
Yaks have a mild nature and usually steer clear of people, but during their mating period, they get aggressive and jealous and sometimes the females spend days picking nits out of their yak-manes.
Great Apes are the same way. You do not want to run into a Great Ape doing the walk of shame during the mating period, I can tell you that much.