Sexual orientation -- whether a person is sexually excited by men or women (or both) -- is largely determined in the womb. In Western culture, at least, women seem more interested in forming relationships than "just" sex. In the pursuit of strong, emotionally satisfying relationships, women may be more willing to experiment with lesbian sex then men.
People don't "become" lesbians. It's biological. You are born with a sexuality. The only choice that is made is whether to accept who you are or live in denial.
Edit: While that answer probably covers what the asker was seeking... technically it is not accurate or correct.
The word "Lesbian" does not actually refer to female homosexuality in the form of exclusive mating with only other females. The word "Lesbian" only means that a female will perform a sexual act on another female for whatever reason. This is very common among females in most animal species.
Females who exclusively mate with other women all their lives are Lesbians because they perform sex with women however, not all Lesbians are exclusively homosexual.
Either way... mating behavior between members of the same gender or even with both genders is instinctual behavior and not a conscious decision. Our sex urges are no more a conscious decision to us than our urge to eat or drink. The brain simply emits chemicals telling the animal to do something. The animal does not choose what that is, but simply follows the instructions or not. Just like you can decide not to eat or drink, you can decide who to mate with. Women who seem to suddenly become Lesbians simply stopped fighting their brains urges and do what their instincts tell them to do. This is normal behavior in a sexually repressed society that we live in.
Some women are born lesbian. Some are not.
The same reason people like to be straight: It's who they are.
because the people want to
they have special needs
she is lesbiane
no
She's not-she's married to Homer.
because your mama is a slot and she is a lesbiane