Why wouldn't distilled water boil in a microwave, and why would the fork make it boil?
Water boils when bubbles of water vapor appear spontaneously. These bubbles start very small, and for water to really boil, these bubbles need to grow.
At the boiling point (100 degrees C), bubbles are in equilibrium, and they neither grow nor shrink. Above that temperature, bubbles grow, and at lower temperatures they shrink. Boiling water is actually a little bit above boiling point, but it is really only a tiny difference, because the growing of bubbles (which turns water into vapor) costs heat, so it effectively stops water from becoming a lot warmer than 100 degrees.
But this story is only really true for large bubbles. For small microscopic bubbles, the rules are a bit different, and they tend to shrink faster. As a result, a bubble needs to have a certain critical size to grow. If it is smaller, it will shrink and disappear. For higher temperatures, this critical size is smaller.
The critical size is much larger than a single water molecule, and as a result, bubbles don't really start out of nothing. There needs to be something in the water that makes it easier to form a bubble. That 'something' can be a variety of things, but rough surfaces and dust particles are quite effective.
In the absence of such 'nucleation sites', bubbles don't form, and the water will continue to heat up. It's quite possible to make liquid water of 110 degrees Celsius in this way. When you have water of that temperature, and you add 'nucleation sites' afterwards (a fork, powder or something else), all that built-up energy will suddenly release, and all the bubbles that appear simultaneously can together cause a sort of an explosion.
See also
superheating.