During the E! True Hollywood Story, Pamela Sue Martin admitted fame was a factor for her decision to exit the role of Fallon; she remarked and I quote (as best as I can): “Fame was a factor for me, because for me, that was this defining the sailing of point that was showing me I didn't have my life. It was somebody else's life and it disturbed me.” So Aaron Spelling, while heartbroken, released her to keep her happy. Joan Collins, on the other hand (in the episode), called her "stupid" and that she made a "big mistake".
Al Corley quit because he felt he had done all he could with the role, and was also unhappy they were attempting to move Steven from a gay character to a more bisexual character, and that did not sit well with him as he wanted the Shapiros & co. to explore Steven as a gay man in a relationship with a man, so he opted to exit.