Not a Physicist, so guessing a little. Both seem turbulent/chaotic systems that rely on the previous state. Each part of a dynamic flow/bike trajectory are small fractals of the initial instability/configuration. A bike is designed to steer straight - counter-steering - while moving, so it is unstable around the line/curve. On the other hand, fluid tend to become unstable at the slightest disturbance and if pushed forward, goes into a oscillating state that breaks up into eddies. I would guess that the bike's forward instability gets exponentially higher at lower speeds, and the bike trajectory lines is not pushed forward as with moving fluid above, so the bike shows another turbulent/chaotic fractal system with a different starting configuration. ..I think ;-)