There's a few things that play into that:

1. There's a lot more players coming into the game right now so more competition
2. At the beginning of a new season, you're battling against more people who were high ranked than you do at the end of the season... because the matches are done trying to match you to a similar rank. So the longer the seasons runs, the more you're battling against people that are closer in reality to your rank.

See how it goes as the season progresses :)