Darius Pull Sion Ulti Interaction

Noticed this the other day while playing Sion. An enemy player was getting away and was living by nothing (what's new?) and so I decided to ulti towards said enemy player who was just beyond second bot tower. During this time, a Darius was there and as I was ulti'ing towards said teammate, Darius was able to pull me towards him from BEHIND ME, away from the enemy player while the ulti was up. Then after he pulled me, the ulti either ended or I "hit" Darius and he was stunned. My understanding is that no CC can be placed on Sion while he is ulti'ing until he hits either a champ or some sort of impassable terrain (ex. wall or Trundle E). I am not sure if this is actually supposed to happen or if it is some kind of bug.