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I don't think you can point at a specific action, but the leaked (Podesta, I think) emails seemed to show that the DNC was working to help the Clinton campaign to the detriment of Sanders during the primary. This lead to the resignation of Debbie Wasserman Schultz from the DNC chairman position.

Later, it turned out the Clinton campaign had essentially taken over the DNC far in advance of the primary results which is highly unusual, to the say the least. [1]

[1] https://www.politico.com/magazine/story/2017/11/02/clinton-b...