From what I understand RIOT changed the ranked system this year. They took into account your last seasons mmr as well as your placement matchs. From what I understand, this was to help people not rise/drop incredibly high/low like before because winning your earlier games in your placement matchs has a much higher impact then winning in the later ones.
The main reason why people were upset this season is because a lot of people would get into a specific tier then would start messing around. Maybe not as far as trolling, but they stopped trying to win and stopped caring if they lost, since they knew it was incredibly hard to drop tiers. Since they lost so many games, even though they were say Plat, with all their losses they'd have bronze tier MMR, so when they ranked in this season they got placed in bronze. Now for them to get back where they belong, they will have to work their way back up and hopefully not repeat this behavior again. Also when there was immunity from dropping tiers, I'd run into Diamond 5's during my silver promo matchs.