Until Super Tuesday, Sanders was viable. Those who have no intention to vote for Trump had to choose which Dem best represents their view.
In the Cochran situation, it was a runoff when the dems didn't have one, so the mischief makers had nothing better to do for the runoff. The fact that the Republican elite exploited that situation WILL NOT be forgotten, and the anger will explode if the nomination is stolen from Trump. I'm not saying that as a threat. I'm just taking the "given" and trying to hypothesize the most logical conclusion.
FWIW, I'd much rather see closed primaries, at least to the point where an independent who chose a primary couldn't drop out of their new political party the same day. In the case of the Cochran/McDaniel runoff, it's bizarre that it wasn't restricted to those who were registered Republican for the original primary. I'd also like to see an end to early voting, which skews to the status quo and diminishes the effect of events close to election day.
Until Super Tuesday, Sanders was viable. Those who have no intention to vote for Trump had to choose which Dem best represents their view.
That's speculation not evidence. And one could equally well speculate that Dem voters thought (rightly or wrongly) that Trump with his high negatives was so eminently beatable in November that getting him as the Dems' opponent was the highest priority.