You're generally put into a bracket with a relatively small number of other players based on the time at which you entered an event (by gaining points). This PvP tournament has brackets of 500, for example. So when you win your first match, you get grouped with 499 other people who are winning their first match at around the same time. This means that there's a little bit of a game involved in deciding when to join - obviously you want to enter events such that you end up in a bracket with more casual players.
But then the match-making has to do with a hidden ranking the game keeps for you; you're not just fighting people in your bracket. The game will bring up people who have a similar MMR to you, and they'll be worth an amount of points which depends on your scores in the tournament (or maybe on your rankings in the tournament). You get more points for beating people with more points, and you lose more points if you get beaten by people with fewer points. This biases tournaments towards the last hours, regardless of when you entered - it's really hard to sustain a score >700, but pretty easy to get up to 400. One can game the match-making to some extent by throwing matches in tournaments you're not trying to do well in, by taking a crappy team into a match, retreating, and yielding (skipping) to lock the team in. Then it will show up for other people to attack, and the game will read that as you not having as capable of a roster as it thought.
The PvE events work similarly. The overall Hulk event is unbracketed - that's just every MPQ player, like in a Lightning Round. The sub-events are bracketed into groups of 2000 players. The point value of missions in the event and sub-events seem to be based on something like how many points there are between you and the leader of the overall Hulk event.
