They're concerned about the treatment of unauthorized immigrants, and about protecting voting rights (an issue like campaign finance where progressives are worried the integrity of the political system is at stake — and where the outcome doesn't look good for them).
Actually He has been very critical of the lack of response and overall negligence of congress about this particular issue. If you take a quick look at his Facebook page you can see how vocal he is about issues as particular as Sandra Bland, Eric Garner, and Freddie gray and about issues as broad as the inequality African Americans experience compared to Whites.
Bernie also called out Voter ID laws ,which make voters show IDs at polling places which have consequently put a price on ballot access and eroded turnout – especially among African Americans, young people and recently-registered voters – a non-partisan congressional watchdog concluded.
Bernie Sanders has also recently called out on immigration reform (the article was posted 1 day after the vox one)
Sanders said that as president, if Congress did not pass immigration reform, he would use executive action to give deportation relief to the parents of U.S. citizens, legal permanent residents and so-called Dreamers, the would-be beneficiaries of the Dream Act.
He spoke out against proposals for guest worker programs on Friday, too, but focused more on the exploitation of immigrants who "have been routinely cheated out of wages, held virtually captive by employers who have seized their documents, forced to live in unspeakably inhumane conditions and denied medical benefits for on-the-job injuries."
Bernie is pretty comprehensive under the veneer of a "progressive". Heres an article on his political development