I'm sure the name ocurred to J.K. Rowling because of the Salem Witch Trials in Massachusetts in 1692. Check here:
http://en.wikipedia.org/wiki/Salem_witch_trials That's really the only well known "witch site" in the US. Area 51 didn't happen until around 1930 or so and has never been associated with any magic. Magic and Sci-fi seem to be two very diverse genres except for the rare crossover.
Artemis
