I think the reason is because Juventus is safe enough to be able to get a ticket to the Champions League next season so they don't have any greater motivation for the rest of their matches this season.
It was really surprising when Juventus couldn't win against the Salernitana team who were certain to be relegated this season and even Juventus almost lost, but fortunately there was a goal from Rabiot in the final minutes of the match to prevent Juventus from losing at home.
And there was a lot of player rotation carried out by the Juventus coach because they were looking at the Coppa Italy final against Atalanta, I think this was an influence on Juventus' decline in performance.
I agree Juventus has not been playing well lately because they do not have much motivation and are focusing on another big game against Atalanta. They already know they will be playing in Champions League next season so they might be feeling bit too comfortable. This has made them play less intensely and with less energy. Coach has also change players around to prepare for other game which has affected how well team works together. This is clear from their close win against Salernitana team that will be relegated. This shows that Juventus is struggling right now.