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.