Nami Ult blocked by Yasuo Windwall

The link is a .bat file from the website na.op.gg that opens a replay of the game. At roughly 15 minutes, my team engages the enemy team at Rift Herald and when my Nami ulted it was blocked from an allied Yasuo's windwall. Both the Nami and the Yasuo were on my team. I don't know what caused it or how to replicate it.