Placement matches and MMR reduction breeds toxicity.
Played against an enemy team containing someone who was in their 10th placement match. Their jungle got shat on and they lost even though he went 18-3 and almost carried. He got placed in Bronze 3. We knew all of this because he let us known in caps lock just before throwing his keyboard against the wall and apparently reporting his entire team.
http://www.pics.berliner-bildermann-store.de/ebay/donald_duck_010.jpg
Why would you give the powder keg that's ranked another incentive to be at each others' throats?