Within one generation, the palate size decreased. So that can't be explained by natual selection. Well, unless small jaws killed a huge portion of the population before modern times.
Edit: It's not that small jaws killed anybody, the hypothesis would be that in a pre-industrialized society sexual selective (behavioural) mechanisms favoured well-aligned jaws / teeth but with the rise of globalization stronger selective mechanisms superseded them. This would be mostly due to rapid real living standard + wealth accumulation gains more strongly correlated with other behaviours.