Because when you're a big man, making sure that you win against a kid is the only way to prove it.
Maybe Miami would have won this year if the Mavs had been comprised of 13 year olds. Ha!
Check out this video of LeBron James dunking and knocking a kid down during a game while he was visiting a basketball camp. At least the kid wasn't hurt, and LeBron helped him back up, but it was completely unnecessary.
The only reason why this could happen is probably because the game Knockout doesn't have a 4th quarter.