Several reasons. Your rotors could be warped. Its possible the proportioning valve isn't working correctly. Try a different type brake pad material. Make sure your wheel bearings are not too loose.
have you cleaned and lubed the caliper rails?
One of the most overlooked tasks when performing brake maintenance is the floating pins that the caliper mounts to. Make sure they are clean and well lubricated.
usually either the calipers have dirt in them and need to be replaced or the rubber lines leading to them have collapsed inside and need to be replaced.. really kills the gas mileage uh!! chevy has had problems with rear discs on these vehicles,70%of braking is on the front,30% on the rear. therefore the pistons get little use and stick. there are stainless sliders the calipers move on,you need to clean and lubricate them,this should stop the brakes from dragging. I have a 1999 Blazer and my rear brakes were grinding so I replaced the pads and rotors but had the same sticking problem. They dont offer a rebuild kit for my calipers so I had to replace the calipers also.
They Seldom Go Bad. Inspect For Leakage And Sticking. If Yours Stuck Replace It, And The Brake Hose Also. One Reason We Have Trouble With Calipers Is Not The Caliper But The Brake Hose. The Hose Will Rot On The Inside Not Allowing The Brake Fluid To Flow Back Into The Master Cylinder There For Causing Calipers To Stick. If Yours Isn`t Leaking It Should Be OK. Be Sure To Inspect The Brake Hoses, If They Appear Bad Replace Them. GOOD LUCK