During my last ride I was fortunate enough to enjoy a good headwind on the way back the put me to 25 mph at times and 16-20 mph most of the time. I discovered though that the front wheel was rocking back and forth and my hands were dampening it. If I almost let go, this bike won’t let me go hands-free, it was very pronounced. It centered around the headset. This bike has been to ~40 mph before during the century ride last summer, so a bit of 20 mph shimmy is disturbing.
My guess is that something isn’t tight enough. The bike has just hit the 500+ mile mark and needs the standard tune up. The headset is loose or my handlebar extension is too high or the front wheel is slightly (<1mm) off center with the forks.
Some internet research turned up interesting stories about front wheel shimmy. It's apparently a deadly thing in motorcycles. Some bike frames do it and others don't. Larger frames increase the odds and sometimes tightening the headset alleviates the problem.