Ahhhhh, I then yes as the top comment mentions what's happening is you're superheating the coffee. In order for water to boil, there needs to be something called a nucleation site, or a place for the water vapor bubbles to form in the liquid. If there are no nucleation sites, a liquid can actually get heated to well over its boiling temperature. Then, when we introduce nucleation sites, the liquid boils RAPIDLY and VIOLENTLY. Surface imperfections act as nucleation sites, but glass and ceramic are both very smooth. In the chemistry lab, when we heat liquids in glass, we add a boiling chip to ensure the can boil properly and doesn't get superheated.

So, short answer, try adding the sugar before you put it in the microwave! Or microwave for less time, you are over heating your water