This is freaking cool!
What might be going on is the air under the cap is stagnant, and at ambient pressure or close to it, whereas the air moving over the outside of the cap, even though it's regaining pressure as it moves back, is at lower pressure. High pressure inside/low pressure outside => inflation. That's not necessarily a bad thing;
this article-interview with a GM aerodynamic engineer quotes her as saying,
Quote:
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Tonneau covers for the bed help smooth airflow over the truck, and Bloch says soft covers are more beneficial than hard covers because they form to how the air wants to flow.
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I think you'll need sensitive measurement to see any difference between this and a hard cover, but if you can manage that the results should be interesting as to which is better. I'm not sure which I'd put my money on.