There are the obvious factors like unemployment and the lousy housing market, then there are others like gender and race.
Quian found men were more likely to live at home than women, in part because they were waiting longer to get married and "sons have fewer domestic responsibilities—such as cleaning and cooking—when they live at home with parents." We'll spare the feminist commentary for now, but Quian seems to feel the more guys are let off the hook for these "women's chores," the less of a problem they'll have mooching off their parents.
Check out the chart below for proof:
In terms of race, Quian found that African Americans, Latinos and Asian millennials are more likely to live at home than their white counterparts. Conventional wisdom says "extended family living" arrangements are ingrained in these race's culture, however, there are two other factors at play: marriage and location.
Asian Americans, who saw the sharpest increase in people living with parents between 1980 and 2007-2009, marry later compared with other racial groups. They also tend to live in high-rent cities like New York City and Los Angeles.
For both of these reasons, "doubling-up" makes sense. It cuts the rent (or eliminates it entirely) and offsets the pain of having to shack up with strangers on Craigslist.
The chart below shows how this trend has evolved since the early 1980s: