HUD is counting people with "worst case housing needs," which means they make less than 50 percent of the area median income, spend more than half their income on rent and don't have any housing subsidies. The surge in the number of people in that situation happened because of the convergence of several trends: a foreclosure crisis that forced more homeowners into renting, a longer-term increase in the number of people who prefer renting over homeownership, the loss of income caused by high unemployment, and a drop-off in apartment construction in the wake of the real estate crash.
Since 2011, however, a few of those factors have begun to shift. Foreclosures have started to clear, meaning fewer people are being thrown onto the rental market. Incomes on the lower end have recovered, as unemployment has dropped. More people are actually getting help with their rent, in the form of local or federal housing subsidies. And a hot apartment construction market has added enough supply to ease some of the competition.
People were still forming households at a pretty high rate, and continuing to shift from homeownership to renting, but that wasn't enough to increase the number of renters in real trouble by much.
Of course, HUD cautious that America still has an affordable housing problem -- the number of worst case housing needs is still way higher than it was in 2003. The situation is worse in Western states, and hasn't gotten any better overall for elderly renters, whose fixed incomes weren't boosted by the recovery in employment. But perhaps, just perhaps, we've turned a corner.