Originally Posted by mile high king

That phenomenon is a common occurance on motorcycles. I have had it occur on almost every bike I have owned in certain situations. Here is some info from (gasp!) Wingworldmag.com that I think explains it pretty clearly

The Tank Slapper
The most often complained about handling quirk is a handlebar oscillation that’s referred to by many as a “tank slapper.” This condition, which has a frequency range of eight to ten cycles per second, normally occurs when the handlebars are released while the bike is coasting down through some speed between 45 and 35 miles per hour. The handlebars will behave like a fluttering shopping cart wheel. As it begins, the handlebars begin to wiggle from side to side with the arc widening very quickly. If the hands aren’t quickly placed back on the bars, the bars will oscillate quickly from lock to lock, sending the machine out of control and the rider to the ground. While overly loose stem bearing adjustment will aggravate this condition, it is not, in itself, the cause of it. The degree to which any bike is prone to exhibit handlebar oscillation is linked to the size of the wheel/tire assembly, the rake and trail dimensions, the total unsprung weight of the front end, and the natural frequency of that unsprung weight. It is also affected by tire/wheel runout, tire imbalance, loose steering stem bearings, loose swingarm bearings, loose wheel bearings, loose engine mounts, worn tires, and tire pressure. Regardless of the existence of the above maladies, a bike will not normally go into a tank slapper while your hands are on the bars providing the necessary damping.
Many different makes and models of motorcycle exhibit handlebar oscillation during coast down at around 40 mph with hands off the bars. This is because the frequencies of the oscillation and of the wheel imbalance or runout come into line, while gyroscopic stabilization of the wheel is low. If you insist on removing your hands from the bars in an attempt to produce this symptom, and if you refuse to accept its presence, check and adjust all of the points mentioned above. If all is well but you still aren’t satisfied, you may wish to consider installing a hydraulic steering damper on your bike. However, the last thing you want to do is intentionally over-tighten the stem bearings to provide additional hands-off damping.