I can't say if it was the case here, but enemies can push one another up a steep incline.

For instance, if the enemies see you and all go to the same location trying to get you, they bunch up and can push one onto the steep incline of the wall. Once an enemy gets stuck on a steep incline, as they wiggle around, they seemingly "bug" up the incline.

I built a "gladiator pit" that I would drag enemies into with the harpoon and witnessed numerous times the enemies pushing onto the steep incline and getting out. I was able to prevent this by using the pick ax, as it gets a more vertical wall than the hoe.