I experienced this for the first time last week decending a mountan at 70K/h. Ahh that was scary. I got out of trouble by not panicing, siting up a bit, bleading off speed with the rear brake which did not do anything to stop the wobble so I pressed my knees into the top tube. Bingo, presto, wobble gone. I have done these high speed decents plenty and since and it has only happened the once.
What is the cause?