They mention high mercury levels as a trap or from wine that the emperor drank, but neither is likely (they often used to add lead to wine as a sweetener, but not mercury AFAIK). But, mercury contamination in tombs, especially in Asia, is very common from the heavy use of the deep red pigment cinnabar, also called vermillion, which is mercury sulfide.
Faik the worry isn't from wine the emperor drank (though I think the consensus is he did take a hell of a lot of mercury as medicine believing it'd give immortality). It's the described artistic floor map of China (at the time) with the rivers of liquid mercury suggested as being real by the high mercury readings.
I was going to say "I doubt it was instantaneous"... but I stand corrected
"They suffered slow oxidation giving way to humid saturation due to groundwater seepage for 2,180 years, followed by rapid oxidation and dehydration in 1974 when the vaults were opened and exposed to the atmosphere. The color coating was severely damaged, then aged and peeled off...
Practically every warrior and horse was painted, but having been buried for more than 2,200 years the pigments were so old they began to change just 15 seconds after they were unearthed. Within four minutes the painting layers bound together by pigments became dehydrated, tilted and broke from the surface.”