Why is there a critical speed for a car-trailer system?

I understand there's a certain speed that if exceeded will lead to instability of the trailer and diverging oscillations. But why is this so? Are there any equations that predict this? I also heard that the Navier-stokes equation predicts these kind of oscillations. But is there a logical explanation to why it could happen? (seems counter intuitive as conservation of energy should mean that oscillations dampen).