I've seen something I cannot explain.

You spelled wards wrong.

Just speculation, but i think it was because when ww ulted nami, his character model overlapped the wall, and something similar to how flashing large walls (game pushes you further than originally would've flashed to due to) happens. As ww's model cannot be relocated due to his ult holding him in place, the game instead pushes him "up" to search for an area where the model can actually be positioned. After the ult finishes, his character model is pushed back out of the wall and "falls"