In this video, a chopper was waiting for President Obama, we all know that as a President, he is really a busy person and it can't be avoided that there are times when he forgets something.
Just like in this footage, he was in a hurry to get on the chopper maybe because of an important appointment, because of being in a hurry, he forgot to salute the Marine who's standing beside the chopper.
It was quite obvious that the Marine was shocked with what just happened. It seems that President Obama noticed that he forgot something, so he stepped out of the chopper and went in front of the Marine. He then claimed the hands of the Marine and shook it. It's quite unusual because some people will just shrug it off, but because of his kind heart he decided to come to him and shake his hands.