it seems to me it is purely the interaction effect of the tailplane compressing the air as it gets near the ground.
Sorry, but the "compression of air" you are talking
about is called RAM pressure or
Chord Dominated Ground Effect. This does not occur until about 1 chord altitude above ground which I dont think it's possible to get a Hstb on a 737 - and definaately not at 10 deg pitch. In addition, the Hstab
is probably a symetric airfoil, so the effect would
be minimal!
The aircraft being on the ground is EXACTLY THE POINT! Had it been in the air, the downwash would go BELOW the Hstb..
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