I've had several (three or four) MacBook batteries swell up like that from normal/slightly heavy use after only a couple of years. This was in the 2007-2008 era or so, though, when you could actually remove and replace them.
My father's 2010 MBP did this a couple of weeks ago. He opened it to stick an SSD in it and saw that the thing had bulged inside the chassis. Might have been like it for a year.
I think it might happen more than people think.