Could have been for a number of reasons. It was a big moment in the game and it was important to get it right. Maybe he wanted to give himself a second in his thought process, maybe he was willing to let it go but felt that caused Koch to go inside at an angle due to his right shoulder being pushed down and realised it should be penalised. The act in itself was small but screwed the whole scrum in the end.
Either way it was the correct decision.