Prior to the goal Darmian was out of position, therefore Schneiderlin and Blind took it upon themselves to cover the vacant right back position. Caligiuri made the run on the wide right channel, whilst being tracked by the aforementioned players, and he plays the ball back towards Draxler (who Darmian is marking/covering). Draxler passes to Kruse who assists Caligiuri, who should have been stopped by either Blind or Schneiderlin, but instead had a free run at goal.
Darmians run of play was tracking Draxler, then covering the CB space left by Blind and then shifting back to his RB position, as Blind moved back centrally. In my opinion, the players at fault were Blind and Valencia. Blind for not tracking Caligiuri and Valencia for being Valencia (the RB version).
As the ball is played to Caligiuri, Darmian instantly steps up, which is what they're most probably practicing in training, however, Valencia is so far back that it destroys our attempt at playing the offside.