Vorticity is caused by pressure differentials at flow intersections.
The lowest pressures are on the roof.
If the rear slope is too steep,the flow will want to separate do to too fast a pressure rise.
At separation,everything downstream takes on the low pressure of the separation line.
Higher pressure in the side flow 'see's' the low pressure on the roof and wants to flow there.
Where the two streams comingle,they wrap up into swirling flow as the high pressure side air races into the low pressure top air.
Generous edge rounding helps mitigate vorticity if contours are gentle but can't prevent them if the roof is too steep.
Only lofting the roofline up to the streamline path can kill the vorticity.
Like Kamm and Koenig-Fachsenfeld did.
