Any example? Unless it was completely racist, I feel like a lot of people have been complaining about H1Bs for many years without any problems. Also here on hacker news.

There's been a major shift just in the last few months. Those of us who have been concerned about this problem for years welcome them to the party, but with a bit of an eye-roll, because it's obvious that the shift wouldn't be happening if it were trucking and farming jobs under assault and not tech jobs.