it does so nearly 100% of the time for me. Not random at all. See my thread on the subject:
It creates matches like the one in my example virtually every time unless an AT is involved. The lower your MMR is to start with, the worse it gets- essentially setting up a noob to fail even harder than they actually should be. Typically a matchmaking system, given 4 people, will pair the highest and lowest MMR on one team and the two middle ones on the other team (unless the matching is good enough that the MMRs are all very close to where the distinction becomes irrelevant)