It’s that they promote a kind of climate reductionism, one 
  that carries echoes of the deterministic theories that were 
  once popular during the second half of the 19th century.

  Consider, for instance, John William Draper’s History of 
  the American Civil War, published in 1867. Draper, who 
  served as president of the medical college of New York 
  University from 1850 to 1873, was a professor of chemistry 
  and an architect of the so-called “conflict model” of 
  science and religion, bringing a scientist’s eye to his 
  task. On the very first page of his book, he announced “the 
  great truth that societies advance in a preordained and 
  inevitable course” guided by “uncontrollable causes.” Chief 
  among these was the climate.

Such wholesale migrations have been the cause of many conflicts. Steppe nomads have migrated to Europe on many occasions over the last two thousand years and each time it has led to war. Even within Europe this sort of migration was observed. For example, Julius Caesar fought Germanic tribes who had decided to migrate to Gaul because the grass was greener on that side of the Rhine.

The climate change adepts will argue with you on this one, as human induced (micro)climate change is still considered a climate change.

It is however not the only reason.