Well, to put it simply: industrialization. As our diets changed, so did our eating habits. We started to cook softer foods & eat with utensils.
This led to a big change: we stopped chewing as much. (3/5)
We are the products of both evolution & development.
It is true that our jaws have shrunk over evolutionary time. It is *also* true that more recent lifestyle changes have made them problematically smaller.
So this is likely a "secular trend" — a quicker change that's happening as a result of a changing environment.