I'm from California, so I'm used to a fair amount of anti-immigrant sentiment (especially strong in southern California). It ranges from reasonable objections about crime and drug smuggling due to the porous borders, to the racist and paranoid (fantasies about anchor babies and takeover plans).
Interestingly, this is not an American-only problem. The video below has been making the rounds of the Chinese internet; apparently some Hong Kongers are even harassing Mainlanders on the street by singing the song at them.
Basically, many Hong Kong locals intensely dislike the large numbers of Mainland Chinese who come to Hong Kong legally or otherwise (there is not free travel between the two places; Mainlanders must have a visa to enter, though Hong Kongers can visit China freely). They call them "locusts" and object to their dirty habits, rudeness, loudness, dishonesty, and so on. (To be fair, many of these objections are valid.)
There is of course a political element as well. The government of China wants very much to "assimilate" Hong Kong, which means getting rid of the local culture and replacing it with a Beijing-centric one. Hong Kong has a different language, writing system, and culture (for instance, a free press and a much more vibrant religious scene), and their citizenry unsurprisingly strongly object to these assimilation plans. Hatred for Mainlanders is in part a response to Hong Konger's insecurity about the future of their city and way of life.
It's fascinating how these social problems are so widespread, the poor and pushing trying to improve their lot, and the rich and prosperous trying to defend what they've gotten. (Singapore has a very similar dynamic in many ways.)