Karma

Behind the scenes the servers keep track of how you get put into battles by a system called Karma. Karma means that each time you get put into a battle of your own tier along with lower tiers, you lose a point. Each time you go into a battle with higher tier aircraft, you gain a point. Normally this should level out randomly, but when you've played a lot of battles with higher tier aircraft in a row you'll end up with a high positive score. Once this score goes above a certain level (not sure of the level, I think 6) the matchmaker will force you into a battle with lower tiers. The Karma is now 5, so now you continue with random matchmaking until you go above the treshold again. This also goes the other way around: if you play a lot of tiered-down battles, you are sure to face higher tiers eventually.