“Jingle mail” is honoring contract. A valid contract must leave a way out (this is why a military enlistment contract is not valid). There is a cost associated with taking that way out, and as long as it is paid, the contract has been honored. Returning a house in liew of continuing to pay the mortgage is perfectly valid.

As for the rest of the mess, there is a lot of analysis available online. NonEntity has a point, it is the banks' fault. But only because they, like many others, managed to co-opt state power to their benefit.

Were interest rates artificially low? We will never know, because there was no market and no money involved (a discussion of why US Dollars, Euros, Yen, etc are not money is out of scope for this post). The Fed and other central banks set interest rates on their fiat currencies for purely political reasons. Given the environment there is no way to tell what the correct interest rate should be.

This mess, like many others, is attributable to state power. Untangling which specific aspects of state policy caused it is worse than try to untie the Gordian Knot.