Lose traction when upshifting

I've been confused for a long time why the rear of a merc or a porsche suddenly loses grip when upshifting in corner exits? Is it a setup problem? I basicly run them on default aggressive.
 
Could be the weight transfer combined with the sudden torque. When you change gears you loose the acceleration so you get an inertia shift to the front and then all the power comes back on while less grip is available at the rear tires. If you on the limit of the rears at that moment of shift then this will cause a slide and once the tires are sliding they loose a lot of grip and it becomes a "moment".
 
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