I admit, when I first watched this I was dubious. To begin with, the guy realized he was being SWATted before they got to the room he was in. As well, it takes time for officers to get kitted out like that. I figured that first on the scene units would be officers in regular uniforms doing a preliminary sweep to determine where the threat was and evacuate the building. However, I was wrong. There are all sorts of news reports online confirming this occurrence. Apparently, someone called and said there was an ongoing attack / hostage situation. I don't know how the SWAT team was first through the door. Maybe they were training that day and just happened to be available.
Anyway, as I looked a little deeper into this, it became obvious that the reason the guy playing the game knew he was being SWATted was that this thing is happening all the time to people who spend a lot of time filming themselves playing video games online. With the rise of "internet celebrities" there has also arisen an internet trolling culture which thinks it is hilarious to call the police and claim that there is a major violent crime going on at house and/or workplace of the person. Then the caller sits back and watches as the internet celebrity (most of whom spend humungous amounts of time filming themselves playing computer games) is rousted by the police on live videocam.
Most of the time it's not quite as drastic as an actual SWAT team coming through the front door. In fact, most of the time police seem to respond to the scene, investigate, get confused as the guy tells them that he spends all his time online and someone is SWATting him, and leave after they are satisfied that no one is actually in danger. Here are a couple videos of that scenario (cursing):
If you go to Youtube you can find dozens of these videos.
This is very dangerous. Consider that most of these people are playing games which involve violent content. Most of them will also be wearing headphones so that feedback does not come through their microphones. There is significant potential here for an officer to walk up to a room and hear some sort of in game trash talking or just plain old or cooperative in game discussion. "OK, I'm going to shoot these three. You take care of the other two on the left side." The officer enters the room ready for a firefight, the surprised gamer makes a sudden move and there's an injured or dead innocent.
Of course, by the nature of the beast it is often quite difficult to catch and prosecute the person who does this sort of thing. A fifteen year old in Utah using a disposable phone or a number masking site to so this to someone in Maryland could be almost in impossible for a local police department detective to catch. It's really more of a federal issue, but individual cases would probably be too small for federal response.