Watch it again. No-one is closing down the player in possession to block the pass. Then neither Gomez nor Trent tracks the striker and he is left completely free 1 on 1, and then takes one touch and hits an accurate shot right across the keeper and right in the corner. Adrian had to cover his near and far post with a player 10 yards from him who scored 30 goals in the Championship last season - how on earth is it is a mediocre shot that he should have saved?
Despite the scoreline, our midfield and defence were a little sloppy at times, and had they closed down or been tighter to the Norwich players then Adrian gets a clean sheet.
He could have come out more aggressively and closed the angle better, though.
Here is the ball just arriving at Pukki's feet. Note how Adrian isn't even in shot, given that it was a telegraphed pass:
Then here is the ball at his feet. Look at where Adrian's feet are, right up at the 6-yard line. He could and should be closer to Pukki, which cuts out his visual angle and possibly forces him to change his mind:
Then this is the shot being taken. Note how Adrian has backed off from the 6 yard line:
So he played it very conservative, and gave Pukki the angle and space he needed to get his shot off.
Again, though, he did what most keepers would do - he protected his goal. But the principles of play apply to keepers as much as outfield players, and closing down the angle once the player has broken the offside line and gone 1v1, is a better option than sitting on the line, or half-committing to coming off it, in terms of minimizing goal chances
