Win probability changes slowly with rank because there are so many random factors in each individual match. Unfortunately, it follows from this that frequent and long streaks will occur, and a player’s rank will oscillate widely. Essentially, a player will tend to bounce between the range of where he is nearly guaranteed to win and where he is nearly guaranteed to lose. The range varies from player to player, but +/- 250 SR/MMR is common and +/- 500 is possible. This problem can be analyzed in depth, mathematically (Overwatch Forums).
I’m assuming above that the player is not throwing intentionally. It’s generally hard to fall more than 1000 SR without doing it intentionally. At that point, the player should probably be reported. A ranking system cannot deal with throwing and boosting, only a banning system can.