When you replaced the brake pads, did you also lubricate the slider pins. If not, then they may have seized up causing the lockup. Another possibility is a failing master cylinder not releasing the hydraulic pressure when the brake pedal is released.
Sometimes you can get an ill-fitting set of brake pads if you go with an inexpensive aftermarket brand. I've had pads that I've had to file the tabs on the end in order to get them to fit. As mentioned, the slide pins, as well as the clips in which the pads slide, need to be lubricated to help the pads move back and forth in the caliper mount as the brakes are applied and released.