Not suggesting he's the sole reason, but it doesn't help the defence when the keeper is not dominating the goal box. The entire defence, including the keeper, have a lot to answer for. They need to get their mojo back.
Still not sure why you think he doesn't dominate his box.
He seems to stay back on his line when he should be coming. Often looks jittery in his when the ball is being crossed, especially on set pieces and corners.
The one area of his game i tell my boy to watch and learn from is his one on one ability. Simply class the way he can make himself big. and calmly attack the oncoming player.
Maybe thats what is deceiving, if im wrong, his apparent calmness, rather than marshalling his troop around him.