The hood is probably acting just like an aircraft wing. The slight curve is like the curved top of a wing. Air moves faster over the curved top surface, thus creating negative pressure, and trying to lift the hood up (it's trying to fly!). The more the disruption of the airflow, the more burbles you get in the flow, and the more turbulence you will have (as with a bug shield). Thus the bouncing. There could be slight differences in the hood curvature between different vehicles, depending upon the fit, accounting for why some have it and some don't. The likely reason the bouncing is occurring at the rear of the hood is because that is where the windscreen intercedes and forces the airflow to depart from the smooth "wing," which can also cause turbulence." The upturned winglets you see on some aircraft are there to help smooth the airflow as it separates off the tip of the wing.
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