Conversation so far:
Are there any other interesting aspects about Ed McMahon and his entertainment career?
Ed McMahon and Johnny Carson joined The Tonight Show on NBC. McMahon describes the whole meeting about as exciting as watching a traffic light change.
Why did it fall upon Ed McMahon's shoulders to host the first fifteen minutes of The Tonight Show Starring Johnny Carson?

Question: What caused McMahon to leave The Tonight Show?
Self-contained rewrite: What caused Ed McMahon to leave The Tonight Show Starring Johnny Carson?