Immigration to the US is done by people of different countries in the hope of better living conditions.
What kind of countries have the most immigrants?
Poland has been the largest of the immigrants to the US
Is there a reason why Poland has so many immigrants?
It seems it was because of World War 1 and 2
That's understandable. Poland was a large battlefield for a lot of the world wars and got invaded many times.
All this immigration I think is what caused the great depression. Too much of a burden and resource strain on the government.
In such a short amount of time too. Normal immigration rates wouldn't have had such an impact on the economy.
It originated in the US, starting with a fall in stocks and the market crash, but spread all over the world. 
That was because the US economy is central to the world economy right?