Thanks Boofer. Some interesting information. I would think the probability of butyric acid bacteria in the milk would be low (it's pasturised and homogenized store bought milk), but we'll see when it's cut open. If it is that, then the cheese will be inedible and, presumable, have off smells, etc. Otherwise, and what I suspect, it's the buttermilk producing CO2 and the rind, pressed under whey, was tight enough that none of it is escaping, hence the large swell. That's my story and I'm sticking to it.