It would be the development in agricultural knowledge/technology
Development in agriculture enable hunter/gatherer society to shift their behavior. Before this, they had to keep moving from one place to another in order to seek food to live.
After the development in agricultural knowledge/technology, the society now can stay in one exact place for a long period of time and relied on agricultural product as their main source food.
After living in one same place for a long time, the needs to build things to accommodate their living start to arise. They start to built several things and repeat the process every year. In the end, large cities were developed from the initially small hunter/gatherer society.
This site might help you find the answer.
https://sites.google.com/a/pitt.k12.nc.us/world-history-wwii-online-textbook/rise-of-european-dictators
The immigrants were looking for economic stability
