Perhaps the other rider is an accomplished practitioner of "shoaling" and didn't want to be behind you at the intersection?
"Generally speaking, I don't engage in competition when I commute. (I don't really engage in competition when I race, either--there's nothing particularly competitive about clinging tenuously to the back of the pack.) However, that does not stop other commuters from competing with me. For example, there's an unwritten rule among New York City bike commuters, and it applies to all riders, regardless of age, fitness, or style of bicycle. This rule is as follows:If you stop at a red light and there is already another cyclist waiting at it, you must stop your bicycle in front of the rider who is already there.
As far as I know, I am the only cyclist in New York who does not observe this rule, because while I'm quite happy to queue up behind somebody at an intersection, I have never, ever had somebody stop behind me. If you're waiting, someone will pull up ahead of you. If a third person comes, they'll roll ahead and stop in front of the second person. On a busy day, this accumulation results in sort of a shoal of cyclists
which juts out into the middle of the street like a sandbar of idiocy."http://bikesnobnyc.blogspot.com/2009/05/indignity-of-commuting-by-bicycle_12.html