Why did "nativism" reemerge in the 1850s in the form of the "Know-Nothing" political movement?
Nativism emerged in the form of the Know-Nothings during the 1850s because the previous decade had seen a boom in immigration. The immigrants had come from Germany (in part due to political upheavals there) and from Ireland (largely because of the potato famine). The nativist Know-Nothings were most strongly opposed to the Irish but also opposed the Germans.
The major concern of the Know-Nothings was religious. Essentially all of the Irish immigrants and many of the German immigrants were Catholic. The Know-Nothings were strongly opposed to the Catholic religion. They felt that the Catholic faith was inherently incompatible with a democratic system. They felt that Catholics would slavishly obey the commands of their church hierarchy instead of thinking for themselves. They believed this would allow foreign elements (the church leadership) to have too much political power within the United States.
Largely for these religious reasons, nativism arose in the form of the Know-Nothings (also known as the American Party) in the 1850s.