Stopped at stop sign then sometimes i start moving forward what could be causing this

Have your breaks and master cylinder pressure checked. maybe your loosing pressure and the brakes are slipping, if it moves and you let off brake and apply again does it hold the car ? On automatics i Have seen a torque converter malfunction that applies torque to trans which would apply pump pressure to trans wanting to make the car very hard to hold,possibly stalling the car out, Possible but uncommon.