Braking at green lights?

Anyone care to explain to me why so many people brake just as the enter an intersection when the light is green and there is no traffic in front of them? I could almost understand if they did this before getting to the intersection, when they might possibly be able to stop if someone suddenly ran the red light, but I’m seeing people do it as they enter the intersection, or from 15-20 feet away from the intersection. Seems a bit late to do any good, and is a bit annoying.