When you release the brakes, the piston in the caliper backs off a little. There is very little pressure between the pads and the rotor. There is often actually a very fine gap you can see or stick a feeler gage into.
When there is too much pressure (or too little clearance) between the pads and rotor, the heat builds up fast and the clearances gets even smaller, the disc gets hotter etc. It will sure enough grab or lock up.
Jack the wheel off the ground, it should turn with only moderate resistance (you should be able to turn it with one hand). If the pads are hanging much at all, it will drag significantly and be rather hard to turn.
Then figure out why it's dragging. The piston isn't backing off like it should (it's sticky). The caliper is sticking on the rods (caliper bolts). The pads are hanging up and sticking on the guides. The low spots on the rotor aren't lining up with the high points on the pads, sand them with 100 grit sandpaper, across the grain or sideways. Sanding the discs and the pads, deosn't hurt a thing, just flattens the high spots some and freshens things up a little.
Some more exotic things that can cause problems are a kinked brake hard line or a damaged rubber brake line. On some of the braided steel lines the inner silicon sleeve can kink or degrade. There was a recall on many after market stainless steel braided brake lines awhile back, because the liner was getting soft and failing.
If the disc isn't sitting on there pretty straight (dirt or rust between the disc and hub) it can rub fairly hard on a contact patch, with every revolution. One reason they use a torque wrench to tightenthe lug nuts, is so the rotor doesn't get canted (or warped) from unequal pressure on the lug nuts.
If a disc gets rusty, from sitting for awhile without use, it can stick signficantly. The rust will wear off after some use. They can rust pretty quick (overnight), after being cleaned with brake cleaner.