Discontinuities drive strategic thinking by radically shifting the landscape of performance space. The four most often though about: technological, regulatory, consumer needs, financial. Here's a nice example of a relatively slow but massive shift in consumer needs:
"...energy intake from such drinks in the United States increased 135 percent between about 1977 and 2001. Over the same span, energy intake from milk -- a far more nutritious beverage -- dropped 38 percent"