McGarrahan is running at full speed toward the receiver and 2 steps away he is pushed in the back towards the receiver... Now he is stubling forward at a high rate of speed unable to change directions in time to avoid a collsion, What would you do? You can't avoid hitting the receiver, and if you don't protect your own body you could get hurt, so what you do is duck your head and lower your shoulder to protect yourself... If McG wanted to level him harder, he certainly could have...
That being said, I think the refs still got the call wrong... There should have been a flag on the guy who pushed McGarahhan in the back for an illiegal block in the back. The ref said it himself, no flag on McG because he was pushed in the back by the defender, well hello isn't that illegal?