The intermediaries in this tale are a fungus, Batrachochytrium dendrobatidis, and the African Clawed Frog, Xenopus laevis. B. dendrobatidis infects and kills frogs, newts, salamanders, etc.. It has spread from Africa, around the world, in the last 75 years. What spread it was the worldwide trade in a couple of frog species: the African bullfrog, and Xenopus. The bullfrog is a pest spread by people who for some unaccountable reason consider frogs not only edible but a gourmet item, but evidence seems to point to Xenopus as principal culprit.
Who spread Xenopus around the world? Why, it was us (scientists, I mean). Scientists have long used amphibians to study developmental biology -- frog eggs develop into tadpoles in the open, and so can be easily studied -- and since World War II, Xenopus has been the organism of choice. It's featured in every modern textbook of developmental biology. It was also used in the 1950s in medical research and pregnancy testing. Large number of frogs were raised and shipped around the Earth, and the surplus were often sold as pets or released into the wild, where there are now populations spread all over the world. But Xenopus carries B. dendrobatidis, and the die-offs closely follow geographically the adoption of Xenopus as a research animal.
So while all the while we scientists were sanctimoniously lecturing our fellow citizens about how our nasty CO2 was killing amphibians, in fact, we ourselves were responsible. (Well some of us, I personally plead innocence to the sanctimony charge)