Sattva, rajas and tamas are not properties prakriti has; they are what prakriti is made of, in every one of its twenty-three evolutes as much as in its unmanifest root. Sattva is associated with lightness, clarity and intelligibility, rajas with activity, motion and turbulence, tamas with heaviness, inertia and obstruction. In unmanifest prakriti the three are in a state of perfect tension, each checking the others so that nothing differentiated can appear, which is why the unmanifest state is also called equilibrium rather than absence.
Every tattva that follows carries a different proportion of the three. Buddhi, the first evolute, is described as predominantly sattvic, which is why it is capable of discernment; ahamkara divides along all three lines at once, its sattvic aspect producing mind and the ten sensory and active organs, its tamasic aspect producing the five subtle elements, with rajas supplying the energy for both processes to occur. Nothing in the system is purely one guna; the three are always present together, only ever in different ratios, which is part of why Samkhya insists prakriti alone, without any additional principle, is sufficient to account for a physical and mental world of enormous variety.
What actually disturbs the original equilibrium is the question the system answers least directly. Some accounts appeal to the mere proximity of purusha, echoing the magnet simile used elsewhere for their relationship; the classical texts are more interested in describing the sequence that follows than in dating or explaining its trigger, treating the process as beginningless and cyclical rather than as a single creation event requiring a cause.