[70Sixter]

One simple possibility is that the new rear tires are not broken-in and are still a bit slick. Especially if you don't drive hard.

This would give less rear stick and therefore more oversteer since the rear would try to slide out while the stickier (broken-in) fronts hold their own.

I would not make any suspension changes until I got the rears broken-in and played with the air pressures.

You need a good baseline going into changes.