Steam and damp air from the bathroom, washer / dryer or other sources that emit high humidity, can cause the smoke alarm to sound.
Pull the notifier slightly away from the door to the washroom / bathroom. You should also make sure to ventilate well and avoid releasing steam directly into the room where the smoke detector is. If you have an ionic alert placed where water vapor can occur, you may want to switch it to optical. Optical smoke detectors are more tolerant.