Partition+Drift: Cultural belief systems naturally drift as participants create new beliefs and change their minds about old ones. Normally the power gradient → of normalization keeps the whole together, however if that whole is partitioned or otherwise divided into mutually unintelligible segments, the populations in those segments will drift away from each other. Colonies split from their parents under this force. US politics → is also characterized strongly by this effect.
Internecine Cultural Warfare: Cultural belief systems experience friction [2] around their differences and imperfections. Typically small cracks or deviations within the normalized whole are slowly processed and rectified, or are adapted and tolerated. However, these cracks can grow under the force of the friction they generate. If the conditions are right, this may result in a vicious cycle that tears a normalized group apart. Counter-cultures split from their parent under this force.
Forking: If a culture comes up against a challenge against which it is not already prepared to address, this may cause leadership within the culture to split over how to respond. If they are unable to rectify their decisions against each other, or come to an agreement, they may drive a wedge down their power gradient, splitting the population. In our hill with twin peaks example, this would happen if the peaks were to drift apart from each other; the population that was formerly beneath the point of separation between them (and which experienced normalization from their perspective) finds itself cleaved one way or another as the point of separation descends. As some of the population moves one way, and some the other, so polarization occurs.