Why do people creep halfway through the intersection at stoplights?

A guy pulled up next to me at a stoplight this morning. Before the light turned green, he crept about halfway through the intersection. This happens rather often, at least around here. Now, you would expect these guys to take off like a jackrabbit, but they seldom do. In fact, they tend to accelerate rather slowly.

It pisses me off. I generally treat them to a whiff of tire smoke when they do this, but I can’t help wondering why they’re doing it at all.