Why did I develop bipolar when my parents and none of my grandparents have it?

I thought it was mostly genetic, so why did I develop it? I started showing symptoms as a teenager about 16/17 years old. I can't think of anyone in my family that has it. My brother has some issues but I don't know that he has bipolar, so I'm not sure what to think. I want to understand why I developed it so I can know my condition better and how to help myself if I can figure if it was from trauma or something because I did have a very traumatic childhood