1). Latinos come from a culture which is far removed from the traditional understanding of "100% pure Americanism" which developed in the United States as a response to mass immigration in the early 20th century, and has been the default attitude of white supremacists ever since.
In Latino culture, Spanish, rather than English, is the first language. White supremacists are insistent that English should be the only language spoken in the United States. Furthermore, the vast majority of Latinos are Catholic, which, to some—but by no means all—white supremacists, makes them followers of a foreign Pope and therefore not fully American.
2). White supremacists blame the influx of Latino immigrants for unemployment in certain traditional industries. They resent the newcomers for taking away the jobs of white workers by agreeing to work for much lower wages. As with all aspects of white supremacist ideology, there's absolutely no evidence whatsoever to support this claim. But old myths persist and white supremacists feed off the poverty and lack of opportunity among certain sections of the white working class to advance their agenda of hate.
3). White supremacists are fanatical believers in the myth of racial purity. To them, the white race is pure and should not in any way be "diluted" by contact with so-called lesser races. The growing number of Latino immigrants represents a serious threat to this ideal as it holds out the prospect of greater inter-racial mixing.