The sequence of events that led to Klopp being kicked out of the game started several minutes earlier when two players collided leaving a Dortmund player bloodied. That player, defender Neven Subotic, had to come out of the game to be taped up...
UEFA
After he was tended to, Subotic had to wait for permission from the fourth official before he could re-enter the game. At the time, Napoli was attacking and it appears that Klopp thought this process was taking too long...
UEFA
Subotic was finally granted permission, but it may have been too late as he entered the box just as Napoli scored a goal and Subotic never really had a chance to impact the play as a defender...
At this point, Klopp freaked out. The fourth official informed the referee of what happened and the referee sent Klopp off...
UEFA
Klopp did appear to attempt to apologize to the fourth official before leaving the pitch...
UEFA
But understandably, he wasn't very receptive...
UEFA