Firstly, prices have skyrocketed well ahead of inflation even as deeper and deeper financing has become common among purchasers as has increasingly sub-prime lending. An assumption of positive return means a steady supply of purchasers, many of whom find that their investment is wasted as of those who go to college a majority don't graduate in 4 years and a little less than half of them will not graduate at all. And of those who do manage to graduate, half of them end up working in jobs that do not require a college degree.
Now you say that purchasing is starting to trend down? That's right, and because prices are rising while ability to repay has decreased and return-on-investment has dropped deliquencies are rising. Because it's a bubble - and the numbers say so .