[RedDevil]

I have a hunch that it has to do with the rounded edges on your rear bumper.
Usually the air besides it is turbulent and the cruved edge has little effect. But on a strong sidewind that turbulence might flatten out, and the air may bend along the corner (the Coanda effect) causing massive drag.

The rear wheel wells add a lot of turbulence, but shielding them over reduces that and increases the drag caused by the corner.
You can easily tape on a cardboard strip or such and see what the effect is.
I expect that the combination of covering the wheel arches and sharpening the corners will have a bigger effect than the combined effect of both measures tested separately.(*)

I was planning to do this mod on my Insight as it was quite sensitive to sidewind, but the effect disappeared by raising the tire pressure so now it would just be for the gains.
But I have not has spare time for the last 5 months or so.
I have bought a sheet of Lexan (which on second thought I will use for something else) and some aluminum stripping to make support brackets and such... but too many plans and too little time.

Tried some ABA testing the other day, but it is hard in a hybrid; no way to ensure the battery involvement is neutral except for pulling the fuse on the whole system.

(*) Like to add, judging from this photo:
that the edge is quite sharp at taillight height but not at the bumper edge, which corresponds to the wheel arch height...