I've not observed that phenomenon in real life, but if it's true and you're looking for idea, there might be a physics explanation to it. When particles get excited, they tend to dissipates their energy by oscillating about a fixed equilibrium point, and this might be a reason. When a corn grain is popped, a lot of energy is released at a time, and there might be some residual energy causing some agitation up to the macroscopic scale, and the only way particles let go of that sort of energy is through vibration. A more compelling explanation might be the fact that after the massive outburst and release of energy, the corn grain was stretched out apart, and to return to its structural equilibrium shape, it shrinks and sizzles. >It's all down to release of residual energy in the aftermath of the pop.