It is also dependent upon the level of oxygen saturation in the tank. If O2 is high, the o2 given off by the plant won't dissolve into the tank and will sit on the leaves like little "pearls."
It is a sign your plants are reaching maximum growth rates. It is can happen at different light and CO2 rates for different people, but it usually means you are doing everything (light, CO2, and fertilizers) well.
If the bubbles would be floating to the surface not sitting there, it would be called streaming instead.