There are two broad categories of motivation. Careerist and ideological.
Ideologically, a Bernie victory would be a much greater blow to the Democratic establishment than a second Trump term. Bernie Sanders winning the Presidency would show, conclusively, that all the people who have spent decades claiming that the Democrats need to be conservative, that they must appeal to the "middle" and so on, were completely wrong. There will be no reason to continue to listen to such people in the future. Since most of these people have careers and thus income that depends on people considering them Very Wise Sages Who Know How Politics Works, this would be very bad for them. The prospect of a Bernie Sanders victory also terrifies the donor class (and mollifying the donor class with such assurances as "we're capitalists, and that's just the way it is" is a fair description of the career of someone like Nancy Pelosi, who is openly touted as one of the greatest House Speakers of all time in part due to her ability to fundraise...). Bernie Sanders isn't a "real" socialist in the sense that his policies do not really challenge the power of capital in any significant way, but this is a variation on the "no good example" imperative in US foreign policy. Bernie winning the Presidency would open up space for a politics that really does challenge the capitalists, and this terrifies them. They know they've been living on borrowed time since 2008.
Now, I realize I blended the careerism into that paragraph, but this also makes a lot more sense when you consider that the real purpose of the Democratic Party is not to win elections in order to exercise power on behalf of their constituents. Democrats (except Bernie, who is only vaguely a Democrat, and to some extent Warren) never describe what they do in these terms. The real purpose of the Democratic Party is to funnel money from donors to a network of "in" consultants and "wonks." Again, Bernie threatens these arrangements in a way that Trump simply does not. Indeed, a Trump victory allows the Democrats to continue playing the "good cop", demanding that leftists and working-class movements fall in line because otherwise the bad cop (Republicans) will come and start roughing them up.
