Essentially, pronouns aren't the same as gender identity, and sometimes people will change their pronouns as they learn more about themselves and how they relate to their pronouns. For some people, it is as simple as identifying as a woman, and using she/her because that is typically the pronoun associated w/ women. For others, especially those that don't identify with a binary identity, it's often a lot more complicated. There are lots of different reasons why someone would use multiple pronouns, but some people use multiple pronouns because their gender identity or expression vary, some use them because they identify with multiple different identities, and some use them because they just feel right, just to name a few.