Going off of memory but wasn't Bernie winning some early on states here and there? DNC put more energy into stopping Bernie in 2016 than they did Trump during the general because they didn't think there was no way Hillary would lose to Trump.
It would make sense for the party to get behind the populist candidate because they would bring in people outside of the usual voting demographic thats going to show up regardless.
Not just here and there, it was basically neck-and-neck for a while.
Iowa was first, and it was full of massive caucus issues and discrepancies, including several precincts where Sanders got more votes but Clinton was awarded more representatives. In the end, Clinton won by the thinnest margin in Iowa caucus history, but many believe Sanders should have won.
New Hampshire was second, and Sanders dominated.
Nevada was third, another caucus, and was hugely controversial as the caucus chair appeared to accept the wrong result of a voice vote (giving it to Clinton even though Sanders's side was louder), which led pretty much to chaos. Clinton again was declared the narrow victor, but many think Sanders should have won.
For the next month they damn near split the states - Clinton won 18 and Sanders won 16. But Clinton won more of the big states, so she took a decisive lead. On April 26, Clinton won 5 out of 6 states in a big northeast primary day, and after that Sanders pretty much closed up the serious side of his campaign. In the end, he won 23 states.
There was massive controversy after Wikileaks showed that DNC officials were explicitly favoring Clinton, something that Bernie supporters had said all along. Besides their personal favoritism for Clinton, the Clinton campaign had gotten to make certain DNC hiring decisions, had control over DNC press releases, and used DNC and state committees to basically launder campaign funds and get extra money to her campaign. On top of that, DNC officials with connections to CNN were feeding Clinton the exact debate questions ahead of time.
There was also a lot of controversy over debate timings. Sanders had called for more debates (like the Republicans were having), but the DNC kept to a lower number which was seen as a means of protecting Clinton. They also scheduled several of the debates at terrible times - the 2nd and 3rd debates were both scheduled for Saturday night, which is when the fewest people are watching television, and one of those was the Saturday before Christmas. Then the 4th debate was scheduled for Sunday night of MLK Jr. weekend, which is another time when few people are going to be watching TV. Since Clinton had more preexisting support and name recognition, it seemed like a purposeful attempt to keep Sanders from gaining ground the way Obama had in 2008.