you sound like you are curious.. let a person who has thought about this stuff for awhile explain the math...
The
are queing together..
so they grab to random people:

Riot believes that groups are stronger than solo so they try and put them against solo players..
MMR is at the point he is about to get demoted..
and the other people do have trash MMR on the enemy team.. so then they group you guys together.. and say good luck..
also the reason you lost..
your bot lane was garbage.. and i am guessing that you went down there trying to gank and help and ended up dieing ..
when people are getting stomped in lane.. in solo Q... /mute them and focus on winning lanes.
if you get in a game where a lane is feeding hard core or just bad.. you need to let them go and get other lanes going man..