The DSC uses the rear brakes to slow down a wheel that is spinning too fast due to lack of traction, sort of like a limited slip differential. That can cause the rear brakes to wear faster than the front ones.
Brakes also wear due to drag, even when the brakes aren't applies. Maybe something's dragging back there, ever so slightly.
My 2015 535i shows that the rear brakes are wearing about 80% as fast as the front ones. That's high. With Frau Putzer's 2006 Accord, the back brakes wore just as fast as the front. The rears started squealing at 145k miles. The fronts were just about to at 147k miles when I replaced them before selling the car to a friend.
One other thing that could possibly cause premature brake wear would be if you live or drive in hilly terrain quite regularly and use the cruise most of the time. The car applies the brakes to maintain your set speed on the downgrades. Incidentally, have you actually pulled the wheels off to inspect the brakes? I was getting a brake warning a while back, and when I pulled off the right rear (it was indicating rear axle brakes) I found the wear sensor just dangling in the breeze. I reattached it, but the warning remained despite having at least 90% of the pucks left. I then ordered up a new pair of sensors (a front and a rear. there's only one on each axle. ($12 on ebay, German made BAP's, too), replaced the rear sensor, and the warning went away. front axle sensor, incidentally, is behind the left front tire, just fyi.
Yes, it'll use one or both, depending upon just how much it has to hold the set speed. You really don't want the brakes doing all the work on a long downgrade. That's a well known fact in the trucking industry, which I was in for 31 years. That's why there are jake brakes (aka engine brakes) on big trucks. Brakes on fire and 80,000 lbs on a downgrade just ain't no fun at all.
Mine has about 43K on it. When I last swapped summer/winter tires, I looked at the brakes...nearly new. The vast majority of my driving with the car is interstate, long-distance. I use my i3 for most of my running around driving.
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