Caldwell says the blowing is caused by unclean milk collection, or presence of yeast. Since this hasn't happened with any other cheeses I've made, and my practice is pretty clean, I didn't think this was it.

Well that's probably it. Early blowing means there's something in the milk that grows in the cheese and produces gas. It can be any one of the following: yeasts, propionic bacteria, coliforms or closridia. Unless you have a way to do a microbiological analysis of your cheese milk there's no way to know for sure.

Your process may be pretty clean but if something's growing in your cheese uninvited that means it's sufficiently unclean for something to grow in your cheese uninvited.

As to why it hasn't happened with your other cheeses: that's probably because you haven't made enough of them yet. Contamination is a stochastic process. Sometimes the organisms that can spoil your milk are in sufficient numbers in cheese milk that they spoil your cheese. Sometimes the DVI cultues prevail.

Personally, I don't make cheese with raw milk. I buy milk raw and pasteurise it Immediately before making cheese. That's not 100% safe and I've had plenty of cheeses spoil (some because I was using milk kefir as a culture, some because I didn't have a proper ripening environment). The question I keep asking myself, when it comes to raw milk is this: do I know what I'm doing? I've only been making cheese for less than two years, so my answer so far is: "no, I'm still making it up as I go along". So I don't use raw milk. And I've still had cheeses blow.