What is the cause of "speed wobbles"? Update

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.
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/