Migrations, big and small, have causes, so let’s start by looking at them.
War or military conflict, with and without “boots on the ground” is an all too frequent cause. The huddled masses trying to escape from the war torn and terrorised Middle East provide a pressing example.
“Real Estate” or land-grab forced migration is another category, of which the evictions of Native Americans by US governments provide examples. Not all examples are historic.
Politically purposed, forced migration was used in the Scottish Highland Clearances of the 1740s. The UK government forced Scots to emigrate to weaken and punish actual and potential Jacobite rebels. It is possible that the refugee precipitating conditions created in Iraq, Libya and Syria etc. may be similar. To wreck one country may be regarded as a misfortune: to wreck at least three looks like policy.
Religious and ethnic intolerance can be a people mover and divider, as the partition of India into India and Pakistan indicates. Managed bigotry is a powerful political tool.
The consequences of Global Warming are causing increasing numbers to move.
With sufficient perception, will and power all of these human-made migration-causing activities could, at least, be reduced. Prevention is better than cure.