Foreman KO'd Moorer because Moorer was being stubborn towards Teddy Atlas, his trainer. Head cases like Moorer for whatever reason try to show up their trainer by not listening and doing the opposite every once in a while. Which is what Moorer did, and he got KO'd for his troubles.
Having the type of personality that gets defiant or stubborn when someone is giving you the proper instruction is kinda stupid. Moorer wasn't trash when he was in shape to fight. The Moorer that would show up to fights looking like Damon Wayans, from the Great White Hype, with the huge pot belly was trash though.