Originally Posted by AlWakRa

So the rears aren't that hard, it may even be on the soft side, as most divorced setups focused on track have an equal or stiffer rear spring rates compared to fronts. I would suspect camber + toe in. I used to have a lot of toe in rear, around 0.9 total toe in, and the rears were sketchy and love to slide even with semi-slicks, getting it down to stock toe in value made it very sticky even with street tires.

Second guess would be very low rear height which mess the rear geometry.