I wouldn't say that Sanders is doing terribly poorly in South Carolina. I think he's doing about as you'd guess, based on the demographics. It's Biden who is doing really well and in ways that go beyond obvious demographics--like still getting 40% in areas with few black voters 
A different way to frame this: the others really underperformed. It reminds me, in a way, of Cruz in Wisconsin back in '16: the one moment in the GOP race where the state's establishment really didn't want Trump, the voters didn't want Trump, and they knew what to do about it.
There are a lot of important differences that I won't dwell on. The point, though, is that this is the kind of result that you can get as a result of tactical voting. It's also the flip of NH, when Klobuchar surged and 'the others' (Biden/Warren) disappointed equally
