Speed wobbles. 13ollocks describes them well. It is an oscillation caused by a driving force (that can be as simple as road vibration) and a system (Bike, wheels and rider) with a resonance of the same frequency (or harmonic of it). And it can vary from a minor scare to terrifying and high speed crash causing.
The cause is usually a combination of a particular bike and a particular rider. Sometimes simple changes are a permanent fix. Two things a rider should be aware of, one as a frequent cause of the wobbles and the other, usually a quick fix - the death grip on the handlebars; back off! Firm but NOT rigid! And the preventative/quick fix; place your knee on the top tube. That usually has a very large damping effect on the wobble.
I have one bike with the issue. Sadly my best custom that fits and rides like a dream. Recently I did a fast descent that while not absurdly fast, I have had to back off for the wobbles on the past. Tried lifting my butt a little off the seat with my legs a little more tensed, hands firmly in the drops with real weight on them and the bike stayed quiet. Haven't yet gone to the really fast hills trying that but I see that experience as encouraging.