It's called sonoluminescence. No one knows exactly why this occurs, but there are a lot of different hypotheses. One of the most common explanations is that when the bubble collapses, the air inside gets pressurized. Increasing the pressure on a gas increases the temperature of the gas. During sonoluminescence, the temperature inside the tiny bubbles becomes so great that the gas begins to glow.
Another hypothesis is that the collapsing bubble lends energy to prolong the life of the otherwise quickly annihilating photons that are spontaneously generated in a vacuum. Sonoluminescence could also be the product of the way photons can pop into and out of existence; the sudden collapse of the bubble making the photons noticeable to those in the macro world."
