One theory is that the part of the brain that's mostly involved in creativity and general insight is easily interrupted by outside noises, including music. They found the same results among people who like to work with music and those who don't.
They further point out research that shows that good programmers tend to have bigger offices. It's not clear whether the better programmers get jobs at better companies, or whether bigger offices tend to make programmers better. But as Fred Brooks said about this study, ``Who cares what the reason is!?'' Big Offices are obviously a good thing.