Why am I in diamond getting paired with gold and platinum people?!?!

The game just auto matches you with people "of your skill level", if there's no one else on that's diamond rank, you'll just be put in with whatever is lying around. When I was still doing ranked, I had a game where I was silver 2 matched against a premade 3 man diamond squad. My squad was an S2, a B3 and a B2, so you can guess how well that went.