Just normal driving. About 75% of the miles were highway miles. Wow, 6 months on a set of pads? Are you doing daily towing of heavy trailers up and down mountain passes?
I used to own a 2008 Honda Accord which was a new body style model year. After 33,000 miles my front pads and rotors wore out and had to be replaced. Turns out they downsized the rotors from the year before to save money.
There was a Service bulletin on the problem and the dealer offered to pay half of the cost to repair them. This was a big topic for discussion amongst Honda Owners at the time, some were only getting 15,000 miles before having to replace their brakes.
I had the dealership check the back brakes while there and they said they were fine. At 39,000 miles, the rear brake pads and rotors were gone. After much discussion with the Regional Manager for Honda, they agreed to pay half of the cost that one time. After that I was on my own.
I was using this car for work and was putting about 24,000/year miles and decided I didn't want to have to replace pads and rotors every 1 1/2 years and promptly traded it in for a Toyota Tacoma and never looked back. I sold that truck with 95,000 miles about 4 years later and never had a brake job on it.
I realize that my Tahoe is unusual with the brake life but I have known many Tahoe owners of the NBS model years and they all had longer than normal brake life on theirs. I'm sure GM realized their mistake and down graded the brake systems on the later years.