i would suggest that the cars lift at 90deg to airflow because they are essentially poorly shaped wings in cross-section.
the flow would find it very difficult to flow under the car, but over is no problem, thus the majority would tend to do this....
with the sides and cutaways inboard of the cars sides, u get a pressure distribution that could be causing the phenomenon of lift at 90deg angles. i admit that it is a very poor wing shape, but the profile would cause this thus it seems plausible. of course, once it starts to incline to the airflow, it simply continues unabated because there is nothing to stop it.
pic may help elucidate. blue is lower pressure