Front wheel oscillation

1. Speed up a little

2. Slow down a little

3. Clamp the top tube between your knees

Some bikes do and some don't. It will probably occur at a certain speed range when all the right conditions are met. I had a bike that would do it at 24 mph coasting but not under effort-pedaling and it would stop at 25. My current road bike has never done it up to 55 mph descending. My tri bike has not shown any signs but I haven't had it that fast yet.

http://www.sheldonbrown.com/brandt/shimmy.html

has only happened to me once so far.  but i knew what it was immediately and let the bike correct itself.  i don't know what i would have done if i didn't know what it was; probably gone into cardiac arrest.

this happens frequently for motorcycles too.  a guy i worked with once said he almost had a heart attack the first time it happened on his motorcycle, did some research and found out all he had to do was let go of the handlebars and maybe slap them.

When you came out of aero you shifted weight back on the bike.  This caused less pressure on the front wheel  freeing it to wobble.  You probably felt it going up some of those hills when you are up pushing hard on the the cranks and pulling on the bars.

I've done 50 on my steel bike but my carbon shakes at 45mph or so. 

insane.  i've gone 40+ on my road bike, and felt fine.  i've hit 40+ on my tri bike in aero with hed h3's, and i can't get comfortable.  so nervous of what the rider i'm overtaking might do or what might pop out.

descending on my road bike was scary that i would hit curves too fast and end up in the opposite lane.  luckily no cars.

have read that clamping your knees to your top tube will held subside.