Same basic problem in any monoculture "crop". By virtue of having less variety, population and habitat resilience to disease, blights etc. drops to the point where large swaths of area can be impacted by what would otherwise be a minor event. Add in mechanization via transport of these species and boom: virus transport effectively exceeds the population's ability to adapt and/or prevents it from dying out locally. Bear in mind that this whole system is also dependent on factors that cannot be controlled in the short term i.e. climate.
Add in that this is not the first time this has happened; check out how droughts in the Midwest in 2012 and 2013 raised the cost of food across the board because of a poor corn harvest, which led into shortages of feed for livestock. There were people that got rich by trading in pork and beef futures because butchering of the too many mouths dropped prices in the short term, and then lower livestock availability caused the price to jump. The whole system is incredibly volatile.