Any number of reasons. Some people mislabel themselves. Some only do so for the sake of pleasing their dominant partner.
Obviously, she's either sexually submissive or a masochist bottom.
A switch is a person who enjoys both topping and bottoming and/or dominating and submitting. Someone who seems dominant in their daily life may need a break from "reality," but that's not switching. It's strictly a BDSM term. For instance, I enjoy submitting to my dominant partner, and I'm a masochist. I also enjoy topping others because I'm a bit of a sadist. However, I'm not interested in dominating someone on a full-time basis. It's not really in my nature to do so. I'm not a 50/50 type.
Just so you know, not all professionals are prostitutes. You can argue semantics all you want since we're dealing with sexual fetishes, but I find the tone of this post derogatory toward professional women. I suppose dominatrices who do it for free are....sluts rather than whores??? I just don't get it.