Not sure if a bug so i'll post it in Gameplay.

Depends. Did you flash over the wall, or were you just walking by? Either way, I think it's just the nature of playing something with such a huge hitbox. Normal champions are small enough and have small enough hit boxes that they can't aggro the tower, even against the wall. I believe cho's hitbox is so big with several stacks, that it's actually extending into the terrain slightly, and overlapping with the turret aggro range ever so slightly; just enough to make it hit you.

I can't say that's intended, but I can't say it's a bug either. I really have no clue. [sg-lux-2]

Yeah Cho's hitbox can get so big that he'll draw turret aggro over walls. I think it happens with a few other champs as well, but under certain conditions.