As mentioned, The speed wobbles were symptomatic of a harmonic interaction. The wobbles have not returned after adjusting the weight distribution on the bike.
Recently encountered speed wobbles going down a bridge at 40 mph. I am interested in understanding what triggers the wobbling.
My first thought was that this may have occurred from unevenly tightened spokes. Any information regarding this frightening phenomenon is appreciated.
Everyone that rides a light road bike long enough eventually has this happen to them regardless of the bike. It is terrifying and many people never trust their bike again after that, but it's not the bike.
Speed wobbles on a bike are always blamed on the components or frame, but are almost always caused by a harmonic interaction between the rider and bicycle. It is certainly possible for there to be a mechanical problem, but if there were it would show up at slow speeds as well.
This article explains the mechanism
http://sheldonbrown.com/brandt/shimmy.html
The natural reaction when this starts to happen is to grip the bars tighter and that almost always makes the problem worse.
If you play around, you can get any reasonably light road bike to shimmy at lower speeds by "shaking" the handlebars. FWIW, I feel like this is much easier on steel alloy bikes than either aluminum or carbon bikes. I think this is due to the better spring response of steel compared to other materials. This is something that is worthwhile to mess around because you'll learn to recognize the initial oscillation and how to damp it out before it becomes dangerous.
There is a lot of material online about the causes of speed wobbles, but I haven't seen spoke tension as a common cause. It usually something created by, and stopped by, rider position rather than anything on the bike (I know you didn't want to read that).
Just a thought though: wouldn't uneven spoke tension show up in the wheel not being "true"?
In any case, look up Jobst Brandt's book "The bicycle wheel". It's the definitive text about this subject, written by a very meticulous engineer and cyclist.
http://www.amazon.com/Bicycle-Wheel-3rd-Jobst-Brandt/dp/0960723668 http://sheldonbrown.com/brandt/
A stronger and better riding wheel is gained from a more even spoke tension. Might be an idea to take the wheel to a get it re-tensioned. ie. slacking off the tension on the wheel and then re-tensioning and truing.
High speed wobble is a bit of an enigma. Given the speed you were travelling - 40mph - the wheel would be spinning at about 500RPM. The slightest deformation or imbalance in the wheel or tyre could easily cause a "wobble".
Given your mention of uneven tension - it could be this with the combination of speed and rider could introduce deformations in the wheel as it spins.
Hypothesis... As the wheel spins - it is the top-most spokes at the 12'O clock position which are under most load. Those are the 6'O clock position are in least loaded position. If you have uneven spoke tension - I imagine the rim at 6'O clock will be flexing a little more than normal - ie. deformation.
I've never had a wobble on a bicycle, but have experienced it on a motorcycle a few times. I wouldn't expect uneven spoke tension to cause it, unless you have multiple spokes with near zero tension. The first thing I'd check is if your headset is really loose. On a motorcycle, it's usually a problem on the BACK that causes a wobble. Maybe a REALLY loose rear hub, a cracked seat stay. Were you coasting or pedaling?
