To offer a possibly way oversimplified theory: In a typical workplace, sexual harassment and random acts of male chauvinism are constrained by corporate culture, so at least from my experience, women can be somewhat comfortable being themselves as paid programmers.

From my experience in open source projects, the number of trolls trolling is quite a bit higher, especially when the primary channels for communication are mailing lists and IRC, which people have an easier time being trolls on due to anonymity and lack of face to face interaction. Not having a boss to yell at trolls for trolling or not having the possibility of getting cockpunched for saying something stupid might be another factor.