I read an article on this a few years ago. Its been happening long enough now that some of my friends in other countries actually call Harley riders "wobblers". Any way. According to the article it is the soft rubber motor mounts. The motor works like a gyro scope in affect, when the bike is leaned down into a corner the gyroscope affect doesn't allow the shake of the motor to change angles as quickly as the bike can be pushed down so the bike in leaning and the shaking force of the engine is still vertical thus pushing the bike up and down while trying to lean into a corner.

Harley is the only company with this problem because their engine shakes like a paint shaker and the motor is rubber mounted which is the only anyone can sit on one at idle.

I have no faith in human perfectability. I think that human exertion will have no appreciable effect upon humanity. Man is now only more active - not more happy - nor more wise, than he was 6000 years ago. Edgar Allan Poe