The die-off started in the 70s when climate change was < 0.5C. OTOH, the story of globalization introducing pests/pathogens to naive populations is an extremely common one, and there's no reason to doubt it here.

Oh yes there is. From the study just cited: “While a large percentage of these declines are attributable to direct anthropogenic effects, such as habitat loss, a substantial amount (48%) are classed as ‘enigmatic‘ declines with no identifiable cause (Stuart et al. 2004).“

Also: “We have recently shown that B. dendrobatidis widely infects amphibian populations across Europe (Garner et al. 2005), suggesting that introductions of the pathogen are rather more ancient than were previously expected. However, mass mortalities are highly clustered within a relatively few high-altitude areas in Spain and France (S. Walker, M. C. Fisher & J. Bosch 2003–2006, unpublished data). These data are inconsistent with a wave-like front of introductions across Europe driving contemporary epidemics and suggest that environmental conditions influence host–pathogen dynamics in determining the outcome of infection within a site of introduction.”