See I never thought it was a jedi mind trick or Kenobi would have never wanted to get out of there so quickly, as he says they will come back in larger numbers. I think it was some howl that scares them off that he probably learned from living on Tatooine for 20 years. I mean the guys got to wait 20 years for Luke to grow up and has ALOT of spare time, so he learns the force ghost trick and figures out how to scare sandpeople!