I believe he followed his ridiculous wife's lead who also used a double handed hand shake moments before he did.
Man is too arrogant to care about protocol. I understand his limo was blocking anyone from exiting or entering Buckingham Palace and had to be moved. Clueless and embarrassing bunch representing the USA.
His whole attitude seems to scream arrogance, elitism and superiority. The complete opposite from what the media try to paint him with their hero worship.