The man knew he was going to die, and his lifelong dream was to buy a buttload of bicycles. So, he bought 53 bicycles, because thats how old he was, and immediately after purchasing them, he died.
Or, 53 people involved in a bicycle-marathon-race were turning a corner when they hit a man, and the man instantly died. They didnt want to be charged for murder, so all of them ran off, leaving the bicycles behind.
Those are my only two explanations.