She was being cocky and rude on set, but that wasn't the main reason she left. It was mainly her decision to leave. She thought she could make it in film. To tell you the truth I think the show got better after she left. Tiffany Amber Thiesssen came in as Valerie and was pretty much Brenda's replacement(to fill the void) and she was a perfect replacement at that. She added an edgier and more mature link that tremendously improved the cast. Brenda was pretty one dimentional. You never knew what you were gonna get with Valerie. The writing also got better. It is really cool to see how the acting and writing got better and better as the show progessed.
Okay the character Brenda they wrote her off the show to live in London and further her acting career, (she tells Dylan to wait for her &she wont be gone forever and she doesnt come back) But from what i have watched of interviews and read in mags books and ect. They let her go bc she couldn't get along with eny of her cast mates off the camera. Im not saying that that is a 100% true but it is what i have seen on some interviews and read in mags books and ect.