It can be an awful lot of things
-Weather: Does your team play a short passing game and the weather is very wet?
-Confidence/Over Confidence: Is there a sign of complacency in the squad, check your *** man feedback before a game to check this?
-Different opposition needs: Have you taken into consideration the strengths and weaknesses of Sunderland and countered that or are you just using one tactic and hoping for the best?
-Injuries: Have you injuries in your squad or low condition with your players?
-Pre Match Interview: Did you say something in the pre match interview that had a negative effect on your squad
I'm sure there could me more solutions but that's all off the top of my head, these things happen and it is only 1 game.