I've never had it happen, but I've read about on BikeSnobNYC's blog; he calls it shoaling (shoaling rant starts about halfway down the post), after fish behavior, and has griped about it repeatedly.

If it happened to me more than once by the same person, I'd probably mock them as I passed; something about not pulling to the front if you're too much of a wimp to be able to stay there, or how my grandmother takes off from lights faster than that.