What's the theory behind who you get matched with?

I was one win away from my Platinum promotions when I started getting placed on the same team as solo low to mid silver players. I went on like that until I made it back to Gold 3. Now, more times than not, the lower elo players had drastically influenced the game, by whatever reasoning. I was wondering why matchmaking would do something like this. People say it was cause my mmr was low but how could i be that close to plat and still be low mmr? Any shedding of light would be appreciated.