For me, it's really the culmination of this guy's performances over the years, even back in Torino. In the Udinese games, he had a few 'shaky' moments, like that cross that he almost fumbled near the end of the match when Bastoni luckily got in the way of the player who kicked his leg instead of the ball. Similarly the save he made vs. Udinese from Fofana's shot that looked like it was going 5 metres off target that he wasn't able to judge. In the Milan match, I thought the 2nd goal a better goalkeeper could have saved it as it wasn't a very good shot by Ibra, but I agree it's mostly Skriniar's fault.
It's hard to pinpoint, but the guy constantly seems nervous to me.