interesting observation indeed
Fact is, like in doubles teams are decided by when you join. So if you have a few players you like to play with every game, you find when their on and message them to join the game your starting, it shouldnt take long for them to do it and because they are the first to join, they become your team-mates
The next 3 players to join will form the opposing tripples team. Why this often seems to be low ranked players is because most higher ranked players will not play a team game with players they dont know or have no experience with, especially when they can simply start their OWN team game and invite their friends to team.
No real need for anything to be random. sometimes a high ranked player will find a tripples game with 3 high ranked players teamed and invite 2 of his friends to join him in taking on the 3 starters. All in all tripples you find games with the same players over and over for a large ammount of the time.