Might be the weird math in LoL's MMR system. It actually can be against you at times as it is like a bunch of "hills" and depending on where you land it can be an uphill battle, or an easier downhill coast into either tilt city or freelo-ville. At least that is to the best of my knowledge.
Math ins't my strong suit if it is theoretical math, if i can apply this shit to non-theoretical physics i'll better understand it. So far, i think this is how MMR works, in "simple" terms. There are factors other than the math and the queue, but when it come to the queue at least it's the math. Might be placing you in with Diamonds b/c DQ busted, low number of high tier players, and/or player on tilt or loss streak and therefore places diamonds with or against plats.