Why do recessions happen?

Just as a marker for future conversation, note that Krugman and others seem to look at major systemic economic crises -- call them "depressions" -- in the following way:

Depression = recession + financial crisis

Which begs the question (since we're looking at financial crisis a good deal), why do recessions happen? Just as a first glance, have a look at this article in The Week. In short, it's a business cycle.