I don't know what you have ridden before, but these monsters (experience from the '5', KTM 380 and Husaberg 501) are so heavy that there's a lot of weight transfer to the front contributing to headshake when you slow down, exaggareted by engine braking.
So when you back off from WFO, be sure to keep your weight back. Also, the KTM's seem pretty front-heavy anyways, so tweak that front preload stiffer. The best remedy to headshake is a wristful of throttle :)
Also, the geometry, at least on the 380, is such that the front wheel turns pretty easily, so be sure to pull those tubes down to flush with the top t-clamp or even lower if you want to take the risk.