When you boil water in the microwave, a portion of the water in the cup is becoming superheated.
This means the liquid temperature is actually slightly above the boiling point, where it would normally form a gas.
In this case, the boiling is hindered by a lack of nucleation sites needed to form the bubbles.
This never occurs when boiling a kettle, for example, because the presence of the rough surface of the element, as well as the convective stirring from rising hot water, are sufficient to produce proper boiling.
Turbulence in liquids is known to provide enhanced nucleation in other cases, when you pour a cola drink, for example.
The addition of a tea bag and, in the other case, simple movement, sufficed to allow bubble formation.
Even with a large proportion of the water superheated, only a little will convert to steam, as the amount of latent heat required for this phase change is very large.
By keeping the cup still and microwaving for a long time, you could blow the entire contents of the cup into the interior of the microwave as soon as you introduced any nucleation sites.
It is this sometimes explosive rate of steam production that means you should take great care when using a microwave oven.
Superheated liquid can boil explosively if something is added, as in the examples given by your previous correspondent, or if the container is moved.
We have seen a spectacular explosion of a bottle of liquid which had just been removed from a microwave in a laboratory, glass and hot liquid were thrown across the room.
This can be avoided by leaving any liquid that has been heated in a microwave to stand for at least a minute before touching it or opening the door. This allows for slight cooling and for the heat to become more evenly distributed.
We recommend that everyone do this when heating liquids in a microwave, even to make a cup of tea.