It has to do with flex(or lack of flex) in the frame and the bike hitting a harmonic that reinforces its self and gives you a tank slapper(minus the tank*) My old TCR team did that at around the same speed. I put on a stiffer fork and it went away. A more relaxed fork might give the same result. First you want to check your headset, sometimes a loose headset can cause this and sometimes snugging it just a touch tight will act like a steering damper. Check your hub bearings as well.
If it happens to you again, press your thighs against the top tube it will help damp the oscilation.

*Google tankslapper and motorcycle, there is a bunch of stuff about this.