After some time playing around with the Practice Tool I realized that casting R-Tidal Wave on your portrait will cast it in the direction you are facing rather than towards the cursor's relative position to your champion.
This has been the cause of several completely whiffed Ultimates resulting in the loss of kill steals; I knew it had something to do with mousing over your portrait as you cast, but couldn't reproduce it til now. This also functions identically for Ezreal's R-Trueshot Barrage, but not his Q-Mystic Shot, so I think it's just a feature of Ultimates.
Anyway, I don't think there is any useful purpose for this. Please remove casting Ultimates in the direction you're facing when hovering your portrait, or if there is some use to it, give an option to disable it.
######(GifCam wasn't capturing my mouse; I'm just moving my cursor on and off my portrait while holding down R using quickcast w/ indicators)