In all likelyhood, this is a combination of springs and rear downforce. At speed, the rear wing produces downforce. This turns the plane of the car so that the front sits higher than the rear, turning the whole car into a big wing, pivoting around the rear end. Thusly lifting the front end up, and this causes air to get stuck under the front end. This creates a high pressure zone under the front of the car, causing lift. The lift then causes the front tires to have less force pushing down on them, decreasing their grip.
Solutions: Less rear downforce, stiffer rear springs, more rear downforce (not an option here, for obvious reasons), or a larger difference in ride height front and rear (rear end higher than the front, counteracting the aerodynamic forces). Note that the ride height is a band-aid, and the ideal here is to do this just with downforce.
