Heating water on a hot plate is safe, because the hottest point is at the bottom of the pot. A lot of relatively small bubbles appear there without much overheating of the water, because there is a lot of nucleation at the uneven phase boundary steel-water.
In a microwave, the hottest points ar IN the water. The glass does not get heated by microwave (at least not much), and radiates off some heat to the surrounding.
Problem: In clean water, there are few good nucleation points to form bubbles, only some dust particles perhaps. So the water gets overheated rather strongly, and a first bubble that appears can grow a lot before it has cooled its surroundings down to 100°C.
Btw. a microwave does not heat the water uniformly. It forms a standing electromagnetic wave (that's not radiation, stricly speaking) in the oven, like a rope swung quickly between two people, or a guitar string. The wave pattern has several knots, where there is very little heating. That's why the microwave oven has the rotating plate, to generate some uniform heating.