User blog comment:Scooter8pie/Brown Mob and the Colorwood Experiment/@comment-4583259-20111125164817/@comment-4583259-20120115064129

Yep, I had the same thing happen to me too. I had to dig really quick, so I wouldn't lose the brown mob (they'll keep descending lower into the ground) and then I had to wait for it to hop out and fill in the hole.

However I tried again, and I was able to push the brown mob down the corridor. So I guess it's random.