When water is heated it can reach a temperature at which rapid evaporation begins to form bubbles of gas (steam) inside the mass of liquid. As small bubbles join they can form larger bubbles, and so forth. It is not as easy as it sounds creating these gas bubbles and this process is called nucleation. For this to occur there must be some defect in the cup or pot or some hot spot to help aid in this difficult phenomena. This happens fairly readily in a metal teapot heated on an open flame. The tea pot has seams and defects, plus with the fire directly on only one part of the pot you get hot spots. So water on the stove forms these bubbles fairly readily once the boiling point is reached. Microwave water in a smooth mug has none of the factors to help assist in nucleation. Thus no bubbles form even though the water temperature has risen beyond the boiling point. When you disturb the mug or put something in it you provide the conditions for nucleation and the water bubbles form rapidly. It is possible to superheat water in a microwave long enough to cause a huge bubble to suddenly form and force the water out of the mug violently. However, this would take heating the water in the microwave for several minutes after it had reached the boiling point. If you then slowly opened the door and kept your face close to the mug, then yes, this could have happened.