Tube between air gap and trap is clogged. Here is a visual.

That or your trap is clogged and not draining quick enough. The water backs up and ends up in the sink from the air gap. This is a safety mechanism also, gives the water somewhere to the go in the case of a drain failure or clog.

Sometimes you get crap in the air gap. Remove the chrome cap (pull or twist). remove the top of the air gap (usually screw off). Clean out crap. Replace top and cap.