I think umpires sometimes get caught by surprise. I remember that World Series game where Clemens made a pitch to Piazza - who took a swing and broke his bat - and Clemens picked up a piece of bat and threw it at Piazza as he was running to first base.
A more obvious ejection scenario, it is hard to think of. He threw a big hunk of jagged wood at a guy. There was absolutely no disputing it was intentional.
No ejection though. I think the umpires were just caught by surprise.
Some things, you just don't see coming.