This may be a boosting ban. You've either improved massively in a very short period of time, or were boosted to gold. Looks like you were in bronze as recently as early August, and hit gold in early September. Your profile shows some extremely good performances on a few champions while your recent games and overall performances on your more played champions like Amumu and Fizz show some particularly weak performances.
A thing to keep in mind is that most boosters use scripts and other cheating programs in order to speed up their boosts. It's not rare to see a player receive a boost to their desired rank just to be banned at a later date because of the booster using cheating programs.
I may be wrong, and I apologize if I am; this definitely looks suspicious, though.