Wintrading in solo queue is when 2 people, or a group of people, take turns using throwaway alt characters to boost each other. It works because theres literally no safeguards against it and because its nearly undetectable. That should be reason enough to confirm it happens. Claiming that every member of the pvp community is above such things is absurd.
If SE decided to release detailed match history on the leaderboard, it'd probably paint a pretty interesting picture.
