Yukon XL (2000) Brake problem

Does anybody know reasons for rear brake pads wearing out while front brake pads have less than 10% wear? This occurred at 35.000 miles. Vehicle was still under warranty but GM claimed it was normal wear and tear excluded from warranty coverage.
One year and 10,000 miles later still limited wear on front pads (both inner and outer) while front disks have corroded on inside surface only.
Master cylinder or proportioning valve problem?

this seems to be an issue in high salt places. or if the truck is not driven every day.