Point of view: Serendipitous wobble
Consumers are pretty habitual, and their life situations are reasonably stable at least within the medium term, while the mental and physical availability of the brands they buy seldom changes dramatically. Consequently, their behavioural and attitudinal loyalties are pretty stable, even over many years.
That said, any person's mental and physical availability can vary a good deal from one purchase situation to another. There are thousands of situational factors that make it impossible for us to predict what exact brand someone will buy next, even when we have a very good idea of their long-run repertoire (their loyalties). These serendipitous factors mean we see a lot of variation in people's buying, it can look almost chaotic. In addition to this brand-shuffling within their repertoire, there is also random variation in how often they buy the category, plus random fluctuation in their repertoire.