I was on a ride today going downhill. Occassionally, I applied both the back and front brakes to slow. At one point the bike began shaking violently. I thought first I was going to crash and I thought the cause might be a flat. I braked slowly and just prior to stopping the wobble went away. I checked my max speed and it was around 42 MPH.

I was in the drops at the time. I've gone faster without a problem. The road was fairly new chip seal. This was the first ride I've done since moving the seat back. I have ultegra brakes. I didn't notice any wobble in the tire, just the whole bike shaking. It did seem to be the front tire.

Any ideas what might have caused this?