While riding no hands on the handlebars, sometimes the front wheel will begin wobbling ryhtmically which may become violent and dangerous if allowed to grow.
I always assumed this had something to do with the harmonics of the bike, that some vibration is in "tune" with the bike and thus the effect only increases. I've noticed this effect is massive when my city cruiser bike has a large rack on the seatpost but nearly non-existent when the rack is removed. Something about the top-heaviness, maybe, but it feels a lot like a harmonic effect and the effect seems to be bike-wide, not just in the wheel.