TL;DR: A fungus, likely spread by human activity, has been causing a large die-off of other-wise healthy amphibians.

I'll add that there are lots of other causes of mass amphibian extinctions, usually tied to localized habitat destruction and/or climate change: https://blaustein.science.oregonstate.edu/pdfs/Kiesecker_200.... These causes also increase the susceptibility of amphibians to various pathogens/disease.