How did it happen?

In short - neither the Republicans nor the Democrats have a good answer. In longer form, here's the most accessible, reasonable, data-based explanation I have seen yet. It takes the form of an imaginary testimony and a written statement before Congress.

From the imaginary testimony:

I reject the two main partisan narratives of this crisis. The Left wants to blame deregulation motivated by free-market ideology. It is true that poorly-conceived regulation was a major factor. However, the blindness of key regulators reflected not ideology but ordinary bureaucratic information loss. The knowledge that existed inside Freddie Mac, Fannie Mae, Treasury, and the Federal Reserve did not flow up to the leaders of those organizations.

The Right wants to blame overly-aggressive lending to minorities and low-quality borrowers, promoted by Congress and regulators. While it is true that many loans were made that should not have been made, the problem was not the color of the borrowers' skins or the content of their credit reports. The problem was low down payments and a large proportion of mortgages for what the industry calls non-owner-occupied homes or investor loans, and what ordinary people would think of as speculators.