i find that women are taking a word that was previously offensive and turning it into an empowering word. it kind of how african americans made nigger into nigga.
In this case, bitch is traditionally used to describe a woman who has a rude, unreasonable or aggressive nature. This all of course raises the question of "What is 'rude, unreasonable, aggressive?" Does it boil down to "You're doing something I don't like or I am offended easily and you have offended me" therefore you are a bitch (and a bad person)?
After taking the word and removing its ammo, it can mean more positive things like a woman who speaks her opinion, doesn't sit there while shes being insulted, takes action,a woman who doesn't take shit from nobody, ect all of which are not bad things yet are seen as "bad". I think that the general idea is that if they're going to think and have these qualities of a 'bitch' then that is what they will call themselves...because those things aren't bad. Thus turning something "bad" into something good.
Its more common for girls to be taught to say whats on their mind and how they feel and to do something if they don't like the situation they're in whereas in the past; they would be taught to keep quiet, obey their husband, and accept their situation no matter how bad it may be. A girl who gets a divorce from her husband can be seen as a bitch (unreasonable), a girl who works long hours away from her children can be seen as a bitch (unreasonable), a girl who stands up and says literally anything in an accusatory manner can be seen as a bitch (rude and aggressive bitch).
Feminist attorney Jo Freeman (Joreen) authored the "Bitch manifesto" in 1968:
A Bitch takes shit from no one. You may not like her, but you cannot ignore her....[bitches] have loud voices and often use them. Bitches are not pretty....Bitches seek their identity strictly thru themselves and what they do. They are subjects, not objects...Often they do dominate other people when roles are not available to them which more creatively sublimate their energies and utilize their capabilities. More often they are accused of domineering when doing what would be considered natural by a man.