Originally Posted by RKMeibalane
He shouldn't have led with his shoulder, as has already been pointed out. Trauma to the shoulder, either via a fall or a direct impact represents one of the two most common mechanisms of clavicular injury. Although I doubt that Greinke knew this before he got hurt, that doesn't erase his mistake.
Put yourself in his shoes. How would you have braced for that blow?
It's instinctive. It protects your head and spine.