Were you bumping the humpty dance?  If yes, I think the car was just getting down.  If not, I'd vote rotor inspection.  If you very very lightly apply the brakes at speed, do you feel a little feedback in the pedal?
Driller, oddly enough, in my life, I've had a wheel leave a car on the interstate, then a completely different vehicle 10 years later, rolled off turning into my driveway.  Neither event was tied to any recent work either.