In video of the interaction, Trump comes up from behind and then shoves Montenegro's Dusko Markovic to get to the front of the group of world leaders. Trump then adjusts his jacket Markovic appears to be taken aback at first, but after seeing that it was Trump, he smiles and pats Trump on the back.

http://www.cnn.com/2017/05/25/politics/trump-pushes-prime-minister-nato-summit