Could you explain this a little. I don't understand. Usually a bubble is an asset where most of it's value is tied to future expected appreciation. And when

Could you explain this a little. I don't understand. Usually a bubble is an asset where most of it's value is tied to future expected appreciation. And when price growth starts to fall this decreases future price, and then it crashes down to its fundamental value(or a little below). I'm not quite sure how this can happen with student loans. Is the discounted future cash flow of student loans widely different from their price?