Three explanations
UD discusses the unsettling finding that some genes, suppressed during life, start to activate hours or days after death.
Simplest explanation: Synchronized activity no longer matters at that point, so there's no advantage in syncing the activity.
Hardass explanation: Bacteria made the world and bacteria run the world. We are just lively and colorful cars for our gut bacteria. After the vehicle is totalled, the bacteria need to recycle its parts.
Optimistic explanation: Many religions have a well-formed idea of a second existence after death. This existence usually happens in a "spiritual place", but not always. Some religions believe that the second existence is suppressed while waiting for a future event. Messiah, Tribulation, etc. Is this reactivation the vestigial remnant or the preparation for the second existence?