judging by the response here my guess would be "SLIME" settling and solidifiying to some extent. I had a shimmy a year or so ago, mine turned out to be a bad (out of round or out of balance) tire. The dealer replaced the tire and all is well.
My check is to securely support the quad on axle stands and with the engine running put it in gear and slowly increase the speed untill you get the wobble. Check the wheels and tires very closely and if you have a bad tire you should be able to identify the source.