I should probably start with the disclaimer that I've never studied the subject and he's won a Nobel Prize.
But Economics is unique among the sciences in that winners can have diametrically opposite beliefs about the nature of reality. Hayek, for example, shared his Nobel with Gunnar Myrdal who supposedly wanted the prize abolished because it also went to Hayek and Milton Friedman.
First, some background. Two broad schools of thought could be said to constitute the Economic mainstream: the Keynesian and the "non-Keynesian" school (also called the Chicago or Neo-classical school) though they're not as binary about Keynes as the names imply. Now try reading Krugman's article linked above: if you were like me, you'd come away thinking "the Chicago school, what a bunch! Better hang with the Keynesians" since those derps at the Chicago school were obviously responsible for the disaster of 2008. But a suspicion might slowly creep upon you that Krugman doesn't argue for the Keynesian school by cataloging the ubiquity of its successes, or the paucity of its failures. Instead he concentrates purely on the (real and imagined) failures of the Chicago school.
There's no law saying Economics professors writing in national newspapers must be non-partisan. At first reading Krugman seems generous, if guarded, in praising his opponents' more famous achievements in the field. One of those happens to be something anybody with a passing interest in the economy would know: that it was an economist of the Chicago school who both saw the crisis coming and was courageous enough to say it - the University of Chicago's Raghuram Rajan who (in Krugman's words) "presented a paper warning that the financial system was taking on potentially dangerous levels of risk" shortly before the crash at a party felicitating Alan Greenspan. Here we should note that naysayers aren't exactly rare but Rajan was different both in the forum in which he said this and what he didn't say - he did not, as the wags put it, predict ten of the last two recessions.
This made him a rockstar in the popular press, and Krugman doesn't try to hide it. But if Rajan is our oracle, surely we should heed what he has to say about the causes of the collapse - and he certainly has something to say. Krugman, however, doesn't think this is important enough to include because it doesn't line up with his political biases [0].
Rajan pinned a good part of the blame on economic misincentives in the form of Freddie Mac and Fannie May, government-sponsored enterprises (GSEs) with which the US govt guaranteed housing loans to people too poor to afford them otherwise and therefore likely to default at rates that would make the lending unsustainable, introducing a massive distortion into the financial sector.
Now this looks like a pretty big deal, so here's Investopedia on GSEs:
[GSEs] are quasi-governmental entities that were established to enhance the flow of credit to specific sectors of the American economy [...] As stated earlier, government-sponsored enterprises have an implicit guarantee from the government that they will not be allowed to fail. Their aggregate loans in the secondary market make them the largest financial institutions in the US. A collapse of even one GSE could lead to a downward spiral in the markets which could lead to an economic crisis. Following the 2008 subprime mortgage crisis, Fannie Mae and Freddie Mac received federal assistance almost immediately to mitigate the negative impact that the default on subprime mortgages had on the economy [...] Some consider GSEs to be stealth recipients of corporate welfare [...] Some feel the GSEs have an unfair funding advantage over publicly or privately held corporations, and that the amount of GSE debt and their corresponding investment portfolios pose too much risk to the entire U.S. financial system. Others believe the role the GSEs play in promoting home ownership, for example, justify their funding advantage [1]
Bush II expanded Freddie Mac and Fannie May. A Republican president expanded a scheme that many if not most of his partymen would call Socialist if he were a Democrat. Populism in general and support of GSEs in particular seems to be a bipartisan phenomenon, a quirk of US politics whose ripples could unmoor the world economy. It did, in 2008; it could do so again. Omitting something like this in an article about the underlying causes of the economic collapse would be unusual to say the least.
There is another way in which GSEs increase the likelihood of default: it's been long acknowledged that increased demand from easier credit drives up property prices, making repayment even harder, eg: "But the bubble in the housing market is caused by creative housing finance made possible by the emergence of a deregulated global credit market through finance liberalization. The low cost of mortgages lifts all house prices beyond levels sustainable by household income in otherwise disaggregated markets." [2]
So that last source is some random guy running an investment firm. But he acknowledges something Krugman fails to: if you want the government to spend its way out of a recession by artificially increasing the purchasing power of the economically underprivileged through easy credit, you're asking for a market distortion on a massive scale that could lead to an equally massive bubble and, inevitably, the next recession.
NOTES
[0] Krugman wrote his article in 2009 while I read Rajan on GSEs in the book he wrote on the collapse which was first published in 2010.
[1] https://www.investopedia.com/terms/a/agency_debenture.asp
[2] http://www.atimes.com/atimes/Global_Economy/GI14Dj01.html
