What’s going on? The explanation is a little bit of weak demand, a little bit of over-supply, a big crackdown on for-profit colleges, and, perhaps, a subtle shift in culture.

1. The college pipeline is drying up.

Data: Education Department

The United States is running out of teenagers.

For two decades, college enrollment grew and grew, bolstered by the coming-of-age of the enormous Millennial generation. But today, the number of young people going to college is in decline. It’s not that today’s teenagers hate higher education; the share of recent high school graduates going on to college has barely budged. Instead, there are simply fewer recent high school grads overall, due to declining birth rates. More than half of colleges and universities say their number of students has declined.

2. There are too many colleges.

Data: Education Department

In the last three decades, higher education has been one of the economy’s most unstoppable growth sectors. In booms and busts alike, the number of two- and four-year colleges kept rising, increasing by more than 30 percent between 1990 and 2010, according to the Education Department.

But now, schools are closing. The number of colleges in the U.S. has fallen for four straight years, and the rate of decline is accelerating, as you can see in the graph above. (The graph measures institutions that give out federal aid, which is a useful proxy.) For decades, population growth after World War II fed the demand for new colleges. But with a relatively strong economy, combined with political and social pressure to restrain tuition growth, colleges are finding it hard to attract students at an ever-rising price point. Last year was the worst year for school closings this century.

What’s more, with fewer teens going to school, colleges are losing their pricing power and offering discounts to fill the beds and lecture seats. Hundreds of thousands of adults returned to school during and immediately after the Great Recession, using the downtime to buff up on skills to serve them during the recovery. But now adults are working more and studying less. The New York Times reports that at some colleges—particularly small, private nonprofit schools—the discounts are “so deep that, while their sticker prices appear to be rising ahead of the inflation rate, the schools are actually seeing their net tuition revenue decline.”

3. The bubble that’s popping isn’t American colleges, overall—it’s for-profit colleges.

Enrollment at for-profit institutions quadrupled in the first decade of this century, to 1.7 million, at one point accounting for 10 percent of US college students. But they were targeted by education advocates and the Obama administration for their low graduation rates and high student debt and defaults. These schools, some of which ran downright scummy businesses to collect government aid without providing much of an education, often catered to adults trying to update their skills in a fallow economy. So in the last few years, they have faced a double-whammy: a healthy job market took away their student demand, and a federal-government crackdown took away their business.