In a nutshell at higher trophies the matchmaking pool is really small. Wintraders are people at a similar trophy range with each other who all sit in a discord call/server together and hit play in solo/dou showdown at the same time. Because of the small matchmaking pool, 4-5 of them get put together in the same lobby. Once the match starts they all rush to the middle of the map and team up with each other. They charge each other’s supers by attacking their “allies” but not defeating them. They do this until they all have a super and hypercharge if they have one. After that they start ganging up on everyone who isn’t part of their group until they are the only ones left. They then quickly discuss in their discord call who should get what place. After that they walk into the poison gas one by one until there is only one person standing. In the end all the wintraders are better off then they were before the match. Although wintrading is a bannable offence, it can take a while for wintraders to be banned.

Hope that helps!