People above explained the specifics but the general is, it's coded language that racists were using when being overtly racist on social media platforms was pretty much the only bannable offense. They started to get really clever to get around the censors but also flaunt they were doing so. It came along with the alt-right movement that was building just before Trump.

They became an acceptable target to hate, were quickly banned and the people who would use that type of language were forced to platforms that didn't ban the harsher words, like 4chan.