Really at any speed, if the rear wheels lose traction and start to spin, they no longer provide directional stability. Static friction between two bodies at rest with each other is greater than sliding friction. When the wheel is rolling in a way that is synchronized with surface that it is rolling on, you have to overcome that static friction for it to slip sideways. So the surface can exert enough sideways force on the wheel to turn the vehicle sideways or to keep it going straight.
But if the rear wheels are spinning faster than the car is moving, the friction between the tire and the surface is MUCH lower. And it is no longer much easier for the tire to roll along straight than for it to slide sideways. Which is why RWD vehicles tend to fishtail and skid in the snow. I remember some very white knuckle times driving my first car up hills in the snow. (1977 Mercury Capri II)
