I'm guessing it is mostly increasing affluence [+], partially causing a portion size war for food other than prepared-at-home meals (which are a much larger percentage of the US diet in 2014 as compared to 1980), and partially applying somewhat richer standards with regards to what food is fresh enough for consumption. At a certain point of affluence, why have three day old leftovers (even if they're palatable) when that competes with a generous serving of delicious piping hot fresh-cooked food? Why keep half a bunch of bananas which are two days from optimally ripe when you can simply replace them with a bunch of bananas which are optimally ripe? etc, etc

I'd expect global food waste, measured in terms of absolute tonnage, to go up over time as the world gets wealthier and less people, at the margin, have to make the choice between "eat foot which is less than maximally palatable to me" and "go hungry."

[+] "But Americans aren't richer in 2014 compared to 1980." Long story, but I strongly disbelieve this, especially with regards to food.