Okay, here goes: The ranked system takes your normals MMR as your first seed for when you do ranked for the first time. From then on you have a semi-permanent ranked MMR, your ranked MMR then gets pulled down a bit each season so you have to work back up for it. Since you ended S4 on bronze, I'll assume your ranked MMR was -low- then it was further pulled down in S5 (and you played no S5 games to pull it back up) and then it was pulled even further down in S6. So you start your S6 placements with a super-low-tier MMR, you're playing with the worst of the worst low-tier players, that doesn't really help your MMR go up, because beating low MMR enemies doesn't help increase your MMR much.
Your final MMR after these 10 matches is very close to what it was before the 10 matches, which is, SUPER LOW, and as such the game assigns you a SUPER LOW RANK for you to play in.
But don't worry! B5 games are super-funny when you're good, I recommend going silly stuff like attack speed Lulu and blasting through the whole thing with a laugh as your MMR slowly recovers back into a playable position.