It's really that simple.

In eras of easy credit, both creditworthy and marginal borrowers are suddenly able to borrow more. This flood of new cash seeking a return fuels red-hot demand for conventional assets considered "safe investments" (real estate, blue chip stocks and bonds), demand of which given the limited supply of "safe" assets pushes valuations of these assets to the moon.

In the euphoric atmosphere generated by easy credit and a soaring asset valuations, some of the easy credit sloshes into marginal investments (farmland that is only briefly productive if it rains enough, for example), high-risk speculative ventures based on sizzle rather than actual steak and outright frauds passed off as legitimate "sure-fire opportunities."