UPDATE: U.S. Park Police spokesman David Schlosser tells POLITICO his service erred in pushing the reporters back, and stressed that the White House played no role in the move.
"That was strictly the U.S. Parks Police that screwed up – that has nothing to do with the Secret Service of the White House or the Administration," said Sergeant Schlosser. "We had some young officers who, when they were told to move the people back -- which we typically do when we're going to make arrests - they moved the people back a lot further than we typically do. That was a rookie, amateur error and they screwed up on that."
Updated, From Gonz's link