I think the first warning sign after Nevada was that they cancelled rallies in Super Tuesday states to do events in South Carolina, which means they'd been planning to just skip SC. Pretty easy to see that as synecdoche for not competing hard enough for southern black voters, whether it's fair or not.
Caveat is that all losing campaigns sound like absolute dumpster fires after the fact, because campaigns are stressful, fractious, unpredictable things. And Bernie did better in the national polls and first three states than he had been doing throughout the summer and most of the fall, so sticking to his guns looks like a good decision at that point.
Hope there's more of this kind of reporting on the Sanders campaign cause I have a bunch more questions.
