Obesity is certainly part of it, though other countries have had rising obesity rates without their life expectancy cratering like the US.
As the thread points out, what's driving the US's drop is primarily high mortality among the 20-70 age group. So when you combine high numbers of gun deaths, drug deaths, and an unhealthy population that doesn't have universal health care, it creates a disaster.
When you look at income directly (rather than percentile) the difference is even more stark. You really see both how poorly the US is doing and how much abject poverty there is (the poorest of the poor are signfiicantly worse off in the US than England):
Combine that with some of the worst social mobility in the industrialized world, and you have the makings of a pretty ugly society with a permanent underclass. I think the US flirting with fascism is only going to get worse as long as the social and health outcomes for a huge part of the population are so poor (and getting worse).