Quick note: This was during the great depression, a period where more people emigrated from the united states than into the country which kind of puts this in perspective.

I think the reason for this was also that most of the immigrants back then came from German speaking countries or countries affected by war, and since they didn't want to get associated with the former governments in Europe and first and second world war. Apparently they also earned more money with american sounding names which was even more of an incentive to change their names.