It's a steering oscillation due to the latent resonant frequency of the steered wheel of the vehice. duh :lol:
Front end shimmy, usually sparked by a bump while on-road. When its bad the vibrations are kinda self perptuating and you'll nearly need to stop to get it to straighten out. It is caused by a lot of things from unbalanced/out of align tires, to relation of steering components position to where they should be (usually occurs after a lift of over 3"), or wallered out steering parts.