Hey there! Unfortunately, there is not much Bungie can do about leaving players. A Competitive Match does not start until there are 8 players (this sometimes leads to long waiting times for players in a high comp rank). That means that one of the players, that was supposed to be on your team, left before the match started resulting in a 3v4 situation. As Bungie does not have a system like other games (e.g. Overwatch) that simply cancels the game when someone leaves, you are put at a disadvantage without being able to do anything about it apart from playing in a pre-made fireteam. Leaving players however lose their glory for leaving the match and can get a suspension for leaving too many matches.