easy,because there were modern agricultural tools,like tractors,invented in 19th century.These tools increased the productivity in rural areas and made a lot of farmers unemployed.
hence,these farmers had to move from rural to urban areas.
The reason why these tools could make farmers unemployed can be explained as follows:
suppose there were one acre crops and one farmer could manage these crops well without modern agricultural tools.Then suddenly tractors were invented and a farmer could easily manage two acres of crops.Since the total arable land was fixed,less farmers were needed.So they were fired and had to go to cities to try some other jobs...
In fact,all the farmers in developed countries,not only those countries as listed by you,but also others like States,Russia,etc.,were making such move in 19th century.
why do you have such homeworks?Usually these things are written in your textbooks...