The ultimate cause is consuming too much, the proximate causes are various elements of modern culture (i.e. Bigger serving sizes, more readily available snacks, alcohol consumption, etc).
There are a few other variables that could play a role.
(1) Share of physical labor jobs vs office jobs
(2) Increased hormone and pesticide use in agriculture (dropping worldwide sperm counts is an underreported story, might or might not be related)
(2A) Less nutritional content in food - has gone down 60% since 1940s according to some studies; likely from over-farming. Need to eat more to get the same nutrients.
(3) Food deserts (related to (1)) - where healthy food options are rare
(4) Less general socialization (internet makes it less necessary to go outside, etc)
The ultimate cause is consuming too much, the proximate causes are various elements of modern culture (i.e. Bigger serving sizes, more readily available snacks, alcohol consumption, etc).