It presents as a quick (4–10 Hz) oscillation of primarily the steerable wheel(s), and is caused by a combination of factors, including initial disturbances and insufficient damping, which can create a resonance effect.
The initial instability occurs mostly at high speed and is similar to that experienced by shopping cart wheels and aircraft landing gear.
Another explanation is that speed wobble is a Hopf bifurcation, a mathematical phenomenon in which a stable fixed point of a dynamical system loses stability as a parameter changes, giving rise to a limit cycle (a path that a system follows repeatedly, looping back on itself in a predictable pattern, which other nearby behaviors tend to mimic over time).
[2] While wobble or shimmy can be easily remedied by adjusting speed, position, or grip on the handlebar, it can be fatal if left uncontrolled.
"[2] The top five influences on wobble have been found to be lateral stiffness of the front tire, steering damper, height of bike center of mass, distance of bike center of mass from rear wheel, and cornering stiffness of the front tire.