If you are trying to remove someone from your back, it is a natural movement to move your upper arm quickly like Aguero did to " shrug them off".
It is difficult to prove what the referee did or didn't see. His head may be pointing towards the incident but his eyes could be looking to the side?
Additionally the same incident can look completely different from different viewpoints. The media keep showing a viewpoint that seems to show contact with Aguero 's elbow to Reid's jaw but viewed from another angle it is clear no contact was made with his elbow. It annoys me the media keep talking how Aguero contacted Reid with his elbow when it is not true.
Having said this I do expect Aguero to receive a ban, he has been trialled by the media and found guilty.