You could see this happening from the earliest days of the Sanders campaign, especially after Netroots 2015 and when BLM showed up at his rallies. Sanders supporters were calling for BLM activists to be tased. (Details here, and here, video here)

While a few supporters cheered for BLM, there were plenty in that crowd who were fucking despicable. Sanders had a problem of racism among his supporters from the very beginning, and he did very little to curb it. If anything, he thinks the path to victory involves going after poor whites at the expense of racial minorities.