Space magic..... In all seriousness, I personally think that most of the stuff he says is prerecorded into the Crucible's announcement system. When an ordinary event happens, like ammo boxes spawning or a team taking/losing the lead, the system detects that and plays the associated message. Possibly, Shaxx has monitors over every arena, each of which he only glances at for a bit before moving to the next arena. However, when a Guardian in a match pulls off some impressive feat, Shaxx either notices that by himself or is notified through the system and starts ranting about how good that Guardian is.