That is a mystery that no one has an answer for (yet). It is also interesting that another isolated center of agriculture was in the highlands of New Guinea.

A very readable book that explains how agriculture/domestication developed and spread in different regions is Jared Diamond's "Guns, Germs & Steel", though he presents no answer for the simultaneous timing.

I have always suspected that plant cultivation probably started in tropical regions during the Pleistocene but there is very little evidence to support this contention. About the only evidence is the distribution of seedless bananas in Melanesia which had to be carried as seedlings by Pleistocene mariners.