Brake lights turn on by themselves?

All of a sudden there has been issues with a dead battery. Have replaced the battery. Seems my back brake lights “turn on by themselves”. I have tried pumping the brakes, turning the engine on and pumping the brakes, also driving backwards or forwards and nothing shuts them off unless I disconnect the battery. It seems to happen at any time of the day. Next day when I hook the battery up, all is ok. Kind of got my mechanics stumped too.