Why does the game keep upgrading my league?

You move up based on your rank points (you see how many points you made at the end of each game). If you win a lot, or even just score the top damage on a losing team, you earn positive points.

But this was almost my exact rant about a week ago. I kept teetering between Silver 1 and Gold 3, and it seemed whenever I'd get back into Gold, everybody else on both teams was higher than me (Gold 2 & 1, Diamonds), and I was in the exact same spot that you're in. Went from mostly in the top 3 in damage to the bottom 3, and wins also dropped badly. Very frustrating. Now I'm teetering between Gold 3 and Gold 2, but I'm very slowly working my way back up the damage list. I still have some crappy games down at the bottom, but now I'm having more in the middle, and a few even in the top 2. Hang in there, and just use it as a way to improve your game, knowing that there probably isn't going to be much extra Au for a while. Try to find a good clan too, if you haven't already, with other players close to your level to squad with. It's helped my game a lot.