Is something preventing the master cylinder from releasing all the way? There is a feed hole between the reservoir and the bore that is supposed to be open when the master cylinder is released. When you first start applying the brakes, the piston cup goes past that hole and pressure starts to build in the system. If the master cylinder is partially applied, that hole is not open to the reservoir. As the brake fluid heats up, it expands and applies the brakes because the fluid can't go back into the reservoir, which makes more heat, which builds more pressure till they finally lock completely. When you park it, the fluid cools down, the pressure decreases. Another possibility is that some idiot put oil in the reservoir which swells up all the rubber parts.