A lot of the details of the argument are convincing: for instance, it's a fact that at high temperatures the bubonic plague bacillus sits around in a flea's gut doing nothing in particular. But when the temperature drops, the bacillus reacts by clotting up the flea's gut; the flea gets starving hungry and starts to go around swigging madly from every blood vessel it can lay its proboscis on (or whatever it is that a flea uses to swig). Since a really big volcano throws enough dust into the atmosphere to lower temperatures around the world, it seems reasonable to connect a volcanic event in 535 with a plague shortly afterwards. Likewise, it seems reasonable to think that widespread famines can have serious political effects.
But you can push "because" too far. Take the invention of Islam: because of volcano- related droughts and floods, Keys argued, a great dam that sustained the Yemen collapsed; because of that, the Arab world's centre of power shifted to Mecca; because of that the Prophet's forebears rose to prominence... Each individual "because" is fine, but the chain starts to look tenuous. Keys is surely right to argue that events like this volcano affect history; but all sorts of other factors - accidents, the lives of great men - need to be taken into account as well. To say that the world is the way it is because of this volcano is to abuse the word.
More dismayingly, the programme wobbled off at the end into doomsday mode: there are plenty of other volcanoes waiting to topple civilisation. Keys suggested that politicians should be taking this into account, though I don't know how they can do this other than by stockpiling food, and frankly I'd rather they let it lie. In any case, they're far too busy worrying about the doomsday asteroids and global warming and the coming of the next ice age to think about volcanoes.
There are other ways of abusing "because" than stretching it too far: tobacco companies have spent the last 40 years trying to trim it down, so that it doesn't apply to the relationship between smoking and disease, or between advertising and smoking. The second part of Tobacco Wars (BBC1) went into chilling detail about the tactics the companies used - threatening scientists who came up with uncomfortable results, using their clout to get rid of unfriendly politicians. It's hard to prove that any individual politician was ever sacked because the tobacco companies wanted it. And likewise, it's hard to prove that Alex Higgins, reduced here to a shrunken head with staring eyes, has throat cancer because he smoked. But the circumstantial evidence was impressive. "Because" seems about right.