She got pregnant the summer before S4 of Angel and didn't tell Joss about it beforehand (I don't believe she should have to: who knows if it was planned? Besides, it's her pregnancy; he's got nothing to do with it). Joss got pissed and instead of thinking of a plotline that would incorporate the pregnancy and still leave her her dignity, he went with the Cordy/Connor/Jasmine thing. Then he didn't tell her he wasn't bringing her back on in S5. She found out through the press. She found out she'd lost her job. Through the press.

Then came AtS's 100th episode special (which turned into "You're Welcome"). They wanted SMG to come on, but she was either busy or just didn't want to. So they asked Charisma. She wasn't thrilled about coming back, but she thought the character deserved a great send-off, so she agreed to do it but only if they didn't kill of Cordy. She was on set (I believe) for filming and Jeffrey Bell pulled her aside and told her they were going to kill Cordy. She was understandably upset. I believe she wanted to pull out, but she read the script and liked it well enough and decided to do it because she felt the character and the fans deserved it.

I do believe that Charisma was deeply unhappy with the direction they were taking Cordy's character and that she didn't do her best acting in S4 because she was so upset. (She's fabulous in "You're Welcome," though, so she obviously wanted to send off the character in style). Which is not very professional.

But. It sounds to me like Joss was at fault just as much. I know that we'll never know for sure, but from everything I've heard, he virtually punished her for getting pregnant. And especially coming from a guy who calls himself a feminist...that's not cool.

So yeah. I think it was an ugly situation all around and no one acted the way they should have.