James Clyburn Wasn’t Responsible for the Biden Surge. So What Was?

I suppose other people have already done this in greater detail than me, but I want to take a look at what caused Joe Biden’s recent surge. First off, here’s the RCP aggregate of the national polling over the past three months:
The green dot is February 29, the date of the South Carolina primary. Biden had gained a point or two before that, but he only really started to skyrocket shortly after the primary. So it’s safe to say that his big victory in South Carolina was the proximate cause of his early March takeoff.
But what was responsible for Biden’s South Carolina win in the first place? Here’s the polling:
The green dot is February 26, the day that James Clyburn endorsed Biden. This surely helped, but Biden had started shooting up four days earlier, on February 22. That obviously had nothing to do with Clyburn.
So what happened on or around February 21? The only thing that stands out is the Las Vegas debate, which took place on the evening of February 19. The consensus for this debate was that Elizabeth Warren left Mike Bloomberg bleeding on the floor, but that no one else especially distinguished themselves. I just reread the New York Times summary of the debate, and it barely even mentions Biden except to note that he joined Warren in attacking Bloomberg.
So there’s something peculiar here. The conventional wisdom says that Clyburn’s endorsement powered Biden to a big win in South Carolina, and the big win in South Carolina powered Biden to victory on Super Tuesday. But Clyburn endorsed after Biden had started surging. Something else must have started the Biden surge, but the Las Vegas debate sure doesn’t seem like it was a turning point either.
There’s no question that Biden suddenly began moving up in the polls around February 21 or so. But what happened?
UPDATE: One theory is that the key mover was the 60 Minutes interview of Bernie Sanders that aired on the evening of February 22. Perhaps that tanked Sanders and Biden hoovered up his supporters. Unfortunately, that doesn’t work: Sanders went up in South Carolina polling after the interview. Biden went up a lot more, but he was taking votes away from Steyer, Warren, Buttigieg, and Klobuchar.
For now, the debate still seems the most likely cause. That’s a little unusual, since conventional wisdom says that debates don’t move public opinion much, but maybe this was an exception.