The explanations offered for this trend, such as the Great Recession, also miss two important demographic trends: (1) the cohort size phenomenon within the U.S.; and (2) the population growth in developing countries phenomenon [and, yes, I still use that phrase even if the World Bank doesn't]. The birth cohort issue is that in 1960 the young adult population in the US was comprised largely of the small cohorts of people born in the Depression, who were pushed along in life by being too young for involvement in WWII but old enough to be catering to the baby boomers. They were the "Lucky Few" that Woodie Carlson has written about (an excellent book--I recommend it). They were a fluke, not really part of a trend that has somehow reversed itself. And, of course, part of their luck was in becoming adults just before developing countries like China, in particular, created a cheap labor market, building on the drop in death rates produced by the spread of medical technology--especially antibiotics--after WWII, as I discuss in detail in the book and have mentioned in this blog before because it is so important to world history.
Seen in proper historical context, the increase in young adults living at home is part of the global demographic transition, not a uniquely odd event.