Holmlid et al have already written several times that the reaction can also occur spontaneously (albeit at a low rate). Spontaneously means without a laser or any other energetic activation. Example from http://dx.doi.org/10.1016/j.ijhydene.2015.10.072 below:
From:
http://english.pku.edu.cn/news_events/news/research/5335.htm
Physicists observe spontaneous symmetry breaking in an optical microcavity
Light energy (Heat) is stored in a polariton within an optical cavity. Think of the polariton as a form of ball lightning. The light is contained in this container as two counter rotating currents, one going clockwise and the other going counter-clockwise. This arrangement forms a magnetic dipole with the spin of the photons pointing out both the top and bottom central axial pole of the optical cavity. This makes for a balanced magnetic behavior of the optical cavity where the resulting magnetic force of the photons counter each other. In this conditions, the optical cavity is magnetically neutral.
When the laser, spark, or high potential electric field is applied to the optical cavity, the index of refraction inside the cavity changes whereby the counter rotating balance between the two photon currents is changed to favor one spin pole over the other.
As with all quantum mechanical processes, there are uncertainties involved where some optical cavities will have a dominant spin pole condition before KERR effect activation. This is the condition that will produce a weak residual reaction effect.
When the KERR effect is activated, then the spin pole of many optical cavities are adjusted to dominant over a balanced initial condition. But not all the spin poles are optimally polarized.
From the article: "Note that the original state would randomly enter the two symmetry-breaking states, and the CW-to-CCW output ratio could exceed 20:1,” said Cao.
Each time the KERR activation stimulus is applied, the magnetic effect (Monopole) is increased as uncertainty in the dominant spin pole orientation is reduced. But even before the KERR activation stimulus is applied, there is a small incipient imbalance in the spin pole orientation. This initial residual spin pole imbalance is what produces the effect that you are referring to.