They have always had some shittyness to them, especially their HLN channel which had some infamous shows and people like Nancy Grace, Joy Behar, Kathy Griffin, and Piers Morgan. They also have some journalists and reporters who had a clear bias on issues regarding Israel. Glenn Beck of all people was probably one of the first to say they had issues since after he left the channel he talked about how CNN was always wanting him to not talk about certain things in regards to the middle east, Islam, and illegal immigration. There were staff there that were outright hostile to him behind the scenes because of this.
It wasn't until 2015 though that they really went off the rails and started shilling heavily for Hillary Clinton, getting to the point that they became notorious for having "technical difficulties" whenever someone started saying something critical about her. Then the primary debates happened, it turned out CNN colluded with the DNC to give Hillary the questions pre-debate. Then Trump started calling them out and that caused them to go completely psycho and since then they've focused their entire network on trying to ruin him by any means necessary. This has caused their ratings and trustworthiness to plummet into the abyss, yet they don't seem to care.