Conversation so far:
When was Conan O'Brien's departure?
January 22, 2010.
Why did he leave the show?
O'Brien's ratings had collapsed.
Did they try to keep him or was he forced to leave?
Conan was allowed to start working for another network as soon as September 2010.
Are there any other interesting aspects about this article?
The deal also granted him $45 million, of which $12 million was designated for distribution to his staff.

Question: why did his time with them start declining?
Self-contained rewrite: Why did Conan O'Brien's time with The Tonight Show start declining?