What you see is the escape of dissolved gases in the liquid.
Water in a cup in a microwave heats fairly uniformly; by contrast, if you heat on a stove, you hear the water "sing" because you get local boiling of the water that is right next to the wall being heated. During the "singing", dissolved gases are driven out (and they don't really re-dissolve - but the water vapor does, to a large extent). In this sense, conventional boiling is an outgassing operation.
The hot water is supersaturated; the presence of the tea bag creates nucleation points so the dissolved air can escape. That's the fizzing you see.
If you really overdo it with the heating of the water, adding a tea bag can cause full scale boiling - and result in nasty burns. It is usually recommended to heat the water with the bag in it to prevent this.
I decided to do the experiment. I took some tap water and boiled it. After cooling it down, I put it in a glass. I then put fresh tap water in another glass. I put the two glasses in the microwave (at the same time, on a turn table) and ran it at 1100 W for about 1:30 min. I then added a fresh green tea bag to each. The result is shown:
I would say that my tap water doesn't have a whole lot of dissolved gas, but the difference in the amount of little bubbles that show up at the rim of the glass is very visible. Note that the boiled water was still slightly warmer than the tap water when I started (maybe 25 C) and the tea came out a little darker. But if anything, that should have meant more bubbles, not fewer. This little experiment supports my hypothesis. Obviously it's possible to design a more careful experiment (mixing boiled and tap water in different ratios, cooling them to the exact same temperature, and measuring the resulting scum line width and plotting it. A high school science fair experiment waiting to happen...)