Until recently, the phrase "chain migration" was used by just about everyone to describe the process by which immigrants were bringing their families to the United States. It was, and is, a completely benign descriptor. It doesn't infer race, religion, or merit. It's simply about method.
However, someone at Democrat headquarters decided that Dems weren't doing enough to pander to their base. Since their base loves to scream "racism" at the top of its lungs, it was decided that "chain migration" was suddenly a monstrous, dehumanizing, term bigots use to "other-ize" dreamers. Attacks began immediately.