As he has been programmed to do. It happened all the time in the World Cup where teams like Germany and Argentina would use up injury time by having a player drop to the ground like he'd been shot at the 89-90 minute mark, and have the rest of the players keep things going by remonstrating with the ref and their opponents about the severity of the challenge.
This ref was actually not bad on that front. There was Reus on the ground and Mulgrew's red, which must have taken up about 3 minutes, but he still gave us another minute and a half. In the World Cup, a lot of the refs were blowing for time about 30 seconds after the player got back to his feet.