Hawkeye was only too happy to volunteer to help out Borelli if it meant avoiding BJ's ridiculous 'surprise party'. It wasn't even Hawkeye's birthday, so what did it matter if he was there or not? However, the surprise was on BJ who definitely wasn't expecting a party of his own, even though he was the one responsible for it. Meanwhile Hawkeye and Borelli got more than they bargained for on their trip when they returned with injuries to their arms and hands. Still it was better than a surprise party would have been. Far less embarrassing.