The photos were taken on 13 March 2009 at a cemetery in a southwestern province of Thailand (Prachaub Khiri Khan Province).
For Buddhists in Thailand, the burial of the deceased is not as widely-practiced as cremation. Buddhists are normally buried rather than cremated when they have no relatives, or when their relatives cannot afford to pay for the cremation.
Cemeteries in rural provinces in Thailand often run out of space as a result of too many bodies being buried in limited amount of land.
So the Buddhists in Thailand practice a religious tradition called “Lang Pa Cha” (which means “the cleaning and tidying of the cemetery”), where volunteers will dig up bodies that were unclaimed by any relatives and cremate them to honour their spirits in accordance with the Buddhist religious rites. Such a ritual is considered to be a good deed and a merit-making process.
At every “Lang Pa Cha” religious ritual, a large number of unclaimed bodies is always found. (In the case of the photos taken here, 64 unclaimed bodies were found).
To cremate a whole body takes a long time, so only the bones of the unclaimed bodies are cremated. Thus the reason for the dissection of the flesh from the bodies as you have seen in the photos.
The volunteers in this ritual are mostly medical staff or emergency response crews who are the first unit to arrive at accident scenes to save lives (easily identifiable by their blue-and-white uniforms and ID cards).
They are used to seeing dead bodies, and that is why they look nonchalant in the photos.
This was originally circulated as a “cannibal rite”, something Snopes ably disproved, but the reality of what it actually was was too interesting not to do a post on it.
Quoted text via the Royal Thai Embassy in inquiry with Snopes.