If a monster sees a character CURRENTLY STRIKING another creature (other than itself), then it is not prepared to defend against the character, as it knows that the character is engaged in combat. A character can only handle one enemy per attack, so the creature isn't ready to put up a full defense against a powerful follow-through Cleave. They are wary, as they're in combat, but so are all creatures, whether or not they're provoking AoO's. It's like a sucker punch; no way to anticipate it. Imagine watching a fight. If you were in the fight, you'd have your guard up. But it's two COMPLETELY DIFFERENT people fighting, and you aren't expecting a punch. You're cautious, but you don't have your fists up. But then, one of them grabs a chair and swings. They hit their opponent in the head and it follows through, flies out of his hands, and hits you. I doubt many would see something THAT severe coming. Maybe a wild punch, but not a chair (or a sword cutting through a friend). In combat, a combatant is always ready to put up a good defense, but they aren't going to expect follow through on a particularly powerful swing against another opponent, so they can't block it to the best of their ability. Cleave is a chancy ability, a monster has no way of knowing EXACTLY when their friend will drop and a Cleave will ensue. As such, it can't be anticipated and fully blocked.