It looks like the agent in question was supposed to go into the building with the President. In one picture he can be seen standing by the door as the President and Mrs. Bush enter with the Chilean President(?). The story says that the Chilean guards then closed in behind the dignitaries and blocked access to the door. It's probably just a mis-communication, but at this point it's hard to tell.
When it comes to protecting the President's life, there is no such thing as "just a mis-communication".
The Service needs to make sure this never happens again.