That's really cool.... That would be nature's attempt to fix what's wrong with the design
Trying to get closer to that natural teardrop shape. It also shows the importance of the rear end (meaning - downstream features) with respect to aerodynamics.
If you wanted to find the ideal shape nature intended (if nature intended cars :P), build up some light and fluffy snow - enough to cause trouble with flow. Then hit the highway and let the snow blow off.... Of course, that won't work beyond the bumper very far :P
Again, that's really cool - I don't live an a region that snows so I never get to see anything like this on my vehicle :P