I disagree. You must have managed to heat the water in the cup to over 100C. Steam bubbles tend to form on imperfections on the surface of the container the water is in. If there weren't enough imperfections, the water couldn't boil properly (also, microwaves heat the water, rather than the container, so maybe the water in contact with the cup was below 100C, whereas the water in the rest of the cup could have been above this temperature. Introducing the imperfection (spoon) into the water at over 100C allowed it to boil instantly.
The world is going mad. Me? I'm doing fine.