So change from rotating around the axles to rotating around the CP/CofG is obviously ruled out. Jet blast impinging on the R/W? Well if that was a cause, wouldn't it be much more noticeable on a/c with tail mounted engines? Tailplane flying into ground effect: Sounds good to me. But some people have said types with T-tails also have this effect (727). Tailplane getting into the wing-wash? Same as above, what about T-tail types? Maybe we should ask the test pilots forum?