Door was not ajar. Security guard has pass key, as security guards usually do.
Sheriff's comments at the press conference last night:
He [the security guard] was conducting an investigation based on customers calling in, and he was personally attempting to locate what was occurring, he happened upon that doorway because it was ajar, which keyed his interest, but our officers were in close proximity so as you can imagine by the timeline there, soon as they came out of the doorway from the elevator he was there so it happened in a matter of seconds
At another point, the sheriff said that even though the security guard had been shot, he was helping the officers clear people out of the other rooms using his pass key, until the officers insisted he go seek medical attention.
With respect to the SWAT team entering the perp's room, the Sheriff said they first entered by "setting off a breach" not by using a pass key to open the door.
When is a Door not a Door?