> If you would like to advance a theory that this is because of a specific environmental change

It probably is, but it could be anything. For all we know, (joke,) it could be a pesticide with traces present in salads and fresh fruit, or exposure to a cleaning chemical in public transportation.

But the bigger issue is that the people pushing these theories are pushing them on moral reasons; IE, making an unfair claim that colon cancer is on the rise because people have immoral lifestyles. IE, from eating poorly or the multitude of other holier-than-thou lifestyle claims at the tip of their tongue.

Personally, my money is that we find out that regularly holding in farts increases the probability of colon cancer, and that we need a public health campaign to fart in public.