There is precedent for accidental mummification of corpses in sarcophagi and the subsequent 'instant' degradation of their skin once exposed to the air; though historically the total disintegration of the skin has taken hours, not seconds, so there is a little artistic licence in the film:
A 300-year-old burial area, in which two bodies were reduced to skeletons while one was perfectly preserved, has left Chinese archaeologists baffled.
When one of the coffins was opened, the man's face, experts claim, was perfectly preserved.
Within hours, however, the face started to go black, and a foul smell began to emanate from the body.
The skin on the corpse - which has now been taken to the local university for study - also turned black.
The body is thought to be from the Qing Dynasty.
It was unearthed on October 10 on a construction site in a two metre-deep hole in the ground at Xiangcheng in Henan province, central China.
Wikipedia explains the plausible conditions that would lead to this:
In extremely dry or cold conditions, the normal process of decomposition is halted – by either lack of moisture or temperature controls on bacterial and enzymatic action – causing the body to be preserved as a mummy. Frozen mummies commonly restart the decomposition process when thawed (see Ötzi the Iceman), whilst heat-desiccated mummies remain so unless exposed to moisture.
The mummies of Asia are usually considered to be accidental. The decedents were buried in just the right place where the environment could act as an agent for preservation. This is particularly common in the desert areas of the Tarim Basin and Iran. Mummies have been discovered in more humid Asian climates, however these are subject to rapid decay after being removed from the grave.