Except Kerbal popping is rate limited by energy input, there's not an instant of energy flow, there's 150 seconds of energy input, each second increasing the energy, popped kernals absorb less energy allowing the unpopped ones to absorb the incoming energy to each the same state.

If you wanted them to all pop at once you'd need to put that amount of energy in all at once. Not impossible, but not going to happen with your home microwave oven