Typical Krauthammer nonsense.

Actually, the Chinese officials had backed down and allowed the Obama party to use its own stairway, but by that time Obama basically said, "Screw them, I'm using the plane's stairway."  

Why did all this happen?

Because Obama put China's aggression in the South China sea as a main agenda item after it became clear that China was only going to be mildly rebuked.  China did this as empty gesture to protest Obama not letting them dodge the issue.

In the crazy Obama derangement syndrome world of Charles Krauthammer, when the POTUS stands up to Chinese aggression in the South China Sea and makes it a main item at the G20 summit, and China reacts with a silly childish antic, he can only find fault with Obama.  Same for his patsy followers.  Pitiful.