While drought and the water requirements for livestock could have had an impact it is strange that pork was singled out when it uses 1/2 to 1/3 of the water per kilo of dressed weight of lamb or beef respectively.

Yes, you raise a good point and it does seem it would be hard to discern root causes just from the bones left as trash by previous civilizations - should probably have said it could be speculated, though lamb and cattle can survive on a diet that cannot sustain a pig, the pigs would be competing with humans for the same calories.