He came out and would have caught the ball with ease taking the pressure off his team completely rather than his defence simply heading it back to the hearts attack. Instead as he grabbed the ball his defender went in for it with his head and the rest is history..
Keeper may not have shouted or defender couldnt hear the shout due to the crowd noise either way its no ones fault, and its nobodys fuck up either. Just one of those unfortunate things that happen.