Rek'Sai annoyance/clarity

Recently played as Rek'Sai jungle against an Azir mid, good play with nice positioning and good peel from team. I started dying a lot during team fights and not doing much and didnt know what was happening. Discovered Rek'Sai ant go under his wall with her burrowed E. Very confused as to why as Azir's wall is only above ground. Maybe because Rek'Sai can be hit out of her E, so its technically above ground still. But then how can she go "under" walls and obstructions.