Didn't hedgefunds "overlong" the housing market? They were borrowing on margin using housing debt as collateral, but the collateral turned out to be junk, so they got margin called when that collateral tanked (the CDOs), and banks with insufficient liquidity to cover the losses either collapsed or got bailed out.

Essentially the housing market bubble was people going long using worthless collateral.

My understanding is that while the market crashed, certain people capitalised on the bubble breaking and kept shorting until it was ran to the ground.