There was a series of huge droughts that ruined much of the produce and of the grains. They had to sell what little they had in order to survive and make up for losses but they were getting killed by imported goods or by grains from other places of the US that weren't hit so severely. This is why the grain prices started dropping which caused many hardships.
Inca and Aztec Societies were similar in that they both got empires by means of military conquests.
