What you see with the 180 MK G1 numbers is a typical "bell curve." You have 4 different values.
If we put velocity at 0, then BC is 1.0 for any projectile (after all, the referrence projectile, a 1 lb lead flat based 2 radius spitzer iirc, also has the same drop at that forward velocity). As velocity changes projectiles will fly better or worse than the referrence projectile.
BC gets better as velocity increases until it doesn't anymore. This makes complete sense as laminar flow differences are not linear with velocity differences. This is why it is called "aerodynamics" and not "aero linear functions."
Anyways, the reason that the BC of the 180 SMK goes down as it goes over 2800 fps is the actual vacuum area behind the bullet. As velocity increases the area of vacuum increases, the boat tail design is to guide gases behind the blullet to minimize that vacuum.
Whirling and whirling, increasing in velocity, onward and onward into viscosity. An old rhyme that explains how increasing in velocity can cause an increase in "whirling" fluids and increase drag.
So yes, if you push a bullet too fast BC will decrease, just as BC decreases as velocity decreases. It isn't anything but a relative measure to how the referrence projectile flies at those velocities.