Maybe for context could you share any reference article or graph which triggered this question? Increased in relation to what, since when?
Not in the US, but from materials read before, it is a combination of things.
Typical causes of weight gain in an individual is simply more calorie intake than is being burnt off. Across a population you can consider environmental, lifestyle and social factors which may contribute to this.
Higher calorie intake (super-sized portions, highly processed foods with high sugar and fat content, cheap convenient fast foods and drinks, expensive healthy/nutritional foods, growth hormones in meats?).
Less exercise (infrastructure not built for cycling and pedestrians, less manual labour jobs, Netflix, Home delivery services, etc).
Health education, Food Advertising/Sponsorship, Chain loyalty discounts, Low wages (poverty), and Political influences would all play a part.