A transonic shockwave forms when locally supersonic airflow over the wing must suddenly slow to subsonic.

If anyone wants a quick thermodynamics lesson: at low speeds, the kinetic energy of a flow can really only be transformed into pressure (or elevation, but that's going to be small for gasses) - the familiar Bernoulli equation. Because of this, our conservation laws - mass, momentum and energy - have one unique solution in terms of the velocity, pressure, and density of the flow. In higher speed flows, we can also convert kinetic energy into temperature. Because of that, our conservation laws end up having two solutions in velocity, pressure, density, and now temperature, with one solution being supersonic, and the other subsonic.

Mathematically, this is because most compressible flow relations (eg, temperature, density, pressure) scale with the square of the mach number. As you may recall, quadratic equations always have two solutions.

These two solutions are not arbitrary, however. A shockwave produces an increase in entropy, meaning that the process cannot happen in reverse, at least not spontaneously. You always go from high mach number to low, because entropy must increase. That said, some manner of perturbation must exist to trigger the shockwave to form, generally in the form of a favorable pressure gradient.