Dear Lenclut,
I believe that it happens with brand new components that are properly adjusted. The culprit is incorrect design of the fork geometry as stated by Irving himslelf in Motorcycle Engineering, although not pertaining to Vincents specifically. The geometry works well for 99% of all encountered situations and if you use the rear brake every time you use the front brake the geometry works well 100% of the time. There are a few situations that exacerbate the problem and most of the victims of wobbles have found those.
I use the Kawasaki damper, but it failed to prevent a wobble several years ago. I have experimented with the geometry and I think I have made my bike wobble proof. Over the years I have cranked the damper up considerably. The handling has improved tremendously on this bike (the single racer). Before I made these changes I could induce a wobble at will by duplicating the conditions of my first wobble.
I believe that the wobble problem only became an issue after many years of observation. The one percenter's became a bigger and more vocal group because of the wonderful communication offered by the Club. In defense of the designers, the Girdraulic is unsurpassed as a versitile solo as well as sidecar fork in terms of handling. Because the circumstances that lead to a wobble are rare, the vast majority of owners will never experience one. Although I have never found worn components to be at fault, I would be safe and urge propper maintenance and a damper as the best insurance. I wrote an article years ago for MPH about this and I am happy to send it to anyone who has an interest.
David