Having re-read the OP, memories of px states that the Apaches were holding at the threshold - did he mean near the threshold, because the distance from the landing area to the holding point would be significant in determining any turbulence downwind of the helicopters.
However, I remain pretty convinced that the wind-shear and turbulence experienced by memories of px is far more likely to be caused by normal wind gusts - he noted that there was a strong wind and such gusts and lulls are commonplace in those conditions.