When a Sion uses ultimate and charges, if an ally Azir has his ult down and Sion tries to run through it from the rear, Sion is stopped, as if hitting a wall.
Don't know if it works when charging from the front.
That seems to be an issue with Azir's ult in general. That champ is bugged to oblivion honestly.
It would seem that one of the major bugs he has in his ult is that it fails to recognise friend from foe. Which means that sometimes your enemies will walk straight through it and get the movement speed buff while your allies are stuck behind it. In that case, your Sion was just incorrectly detected as an enemy and bounced off.
