#1 Not pressing Cardiff enough
One of the biggest problems in this United team is that they don't press the opposition enough and when they do press, it isn't coordinated. We saw that same problem against Cardiff as whenever United lost possession, they weren't squeezing the away side and letting them pass through the middle.
The penalty that Cardiff earned was through one such counter-attack when United lost possession and allowed the Cardiff side to play out easily through the middle. Against a relegated side, the least that is expected of United is that they close down the opposition early but they didn't do that.
Manchester United's lack of pressing may be down to their lack of fitness or lack of desire but those reasons don't give the players a reason to press less. Manchester United's midfield is guilty of not pressing as a unit. Paul Pogba especially wasn't really making any efforts to chase dowm the ball quickly and many are disappointed by the Frenchman's efforts.
Solskjaer has always stated that he wants his United to be the hardest working team in the league but they are way too off from that right now.