It's a response to the sub-prime meltdown and the tragedy of foreclosures. People without homes while an enormous number of buildings are vacant. Reuters has details.
One sorry point about the article: it features no interviews with squatters themselves, but is almost entirely based on interviews with real estate brokers. This makes it seem like the squatters are manipulating the tragic situation for their own benefit. As if the banks and entities that have foreclosed are not. Some squatters may, indeed, be seeking to profit. But I'm sure there are others who simply need shelter.
To be, as Patrick Chamoiseau has written, is first and foremost to possess a roof.