This is probably not a result Ground Effect.
Chord dominated GE starts at about 1 chord altitude, but
will not be noticable until far less. On most aircraft, the
horizontal stabilizer will never get into GE.
My theory is downwash and wake from the wing and fuselage.
Also, wouldn't the rotation in free air be around the center of lift,
not only CG?
If tailmounted engines on rotate, it could be the blast hits
the ground since the engine cant also is 2-3 deg negative and
give a slight reaction force.
Cheers,
M