What event during 1849 and 1850 caused the immigrants to come to America?

In the aftermath of the failed revolutions of 1848-49 a number of liberals fled to the U.S., especially from Central Europe. However, at that time migration was already in full swing anyway. So, there is room for debate as to how many 'extra' migrants went to America for political reasons at the time. (Large scale migration from the German-speaking lands grew sharply from about 1830 onwards, and is often cited as one of the reasons for the establishment of the Hamburg-America Line (HAPAG) in 1847. This predates the failed revolutions).