Check easy things first. You can disable the hill assist and if the problem persists, you know its not that. Brake fluid level and color are easy. I would check the ABS connections at each wheel to make sure they are seated properly/clean. If those all look fine then i would move to taking apart the brakes.
After your drive, walk around your car and put your hand near the wheels/rotors, if you notice one feels hotter, that caliper could be sticking