Why does sweetener ‘swirl up’ in microwaved coffee, but not in brewed coffee?

Question

If a packet of sweet’n’low or sugar is sprinkled into a mug of freshly-brewed hot coffee then the sweetener sinks into the cup. End of story.

If cold or room temperature coffee is poured into the same mug, then heated up in a microwave oven, and then an identical packet of sweetener is sprinkled in to the mug, the sweetener appears to sink down into the coffee, but then shoots back up to the surface and swirls in a spiral/vortex of sorts.

Why?

In: Physics