What are the current leading theories on what causes a person to be Transgender?

Hi, so I'm a trans woman. Two things real quick, I'm asking this question because as a trans person I'm generally interested in what led to me being this way. I understand that the way I worded the title may be mistaken as being rude or implying people can become trans, and I just wanted to be clear that that is NOT my intention in anyway, I just personally couldn't think of a better way to word it. Thank you in advance for any answers you may leave.