That is genuinely interesting. I would have figured Mr. Bernoulli would pop it up a little bit, like 15 or 20 degrees or something. Very interesting.
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I agree it seems logical to only crack open a little bit. However, I think it is aerodynamics at play. There is probably some kind of bow-wave coming over the front end and the wave is quite high with a low pressure are directly above the hood area. That low pressure area may be a few feet high. It also could be due to a ratio between the front grill area vs. the hood area. A large volume of pressurized air coming through the grill acting on a rather small hood. It was an extremely strange phenomena because I don't remember the hood dropping down when I slowed significantly. But maybe serious deceleration was holding it open at some point!
As we all know, bit of air moving over/under a large surface can lift things of significant size...
https://www.youtube.com/watch?v=zGCnZn2_QGs