My personal theory is that he did one of two things (And possibly both.):
One: He broke with what everyone else was doing and was various famous people while the Fifth World was going on. I doubt Elvis, but wouldn't put Buddy Holly past him. (And there was a Neil Young joke earlier.).
Two: He got pissed off during the down-time period with all the politics of "How we're going to get the world ready for the Horrors this time!" and how everyone had their own plan, and wanted to do their own thing, and fought hard to get everyone to work together. Then, when the dragons awoke (Who IEs don't like to begin with), did the SAME BLOODY THING. Which pissed the Dragons off, because they don't play well with each other, forget IEs. Dunkie probably had some similar ideas, and they got along pretty well. As for the "I was gonna have to kill him eventually anyway" comment... I think that might be more a matter of "Better to have honorable enemies than friends of any type." Certainly fits the mindset of the ancient ones...