As I recall, it began with Cameron doing her loner flaky thing. She didn't want to keep the developers from the company they merged with earlier in the season. Donna falsely told Cameron that Diane said Cameron couldn't fire the other developers. In actuality, while Diane didn't agree with Cameron's desire to get rid of the other team, she felt it was Cameron's and Donna's company, and would respect their decision.
Cameron found out Donna lied, and was upset. Later, when she realized Donna was wanting to go public with Mutiny, she basically told the shareholders to choose between them, and everyone sided with Donna, so Cameron was ousted.
When the IPO didn't go as well as hoped, Donna joined Diane at her VC company and Cameron went to Japan and developed games.
Later, when Joe came up with the idea for Internet play, Donna tried to sell the idea to Cameron, saying they could dump Joe. Instead Cameron turned the tables and iced out Donna.