This is actually not just a bug with Riven, this is a bug with self casting.
See when you casted your E, your mouse was on your champion icon, this will sometimes glitch out and do some weird things like what happened to you.
I thought this was a bug before someone informed be about the self casting bug, I will leave 2 YouTube links of 2 cases where this happened to me.
https://www.youtube.com/watch?v=xAnElBha2XE
^ This one is where my ultimate as Ezreal goes in the complete wrong direction, similar to what happened to you. (Notice how my mouse is hovering over my champion icon as I cast it)
https://www.youtube.com/watch?v=mMsrwzvFL84
^ This one I completely flash in place if you look carefully. (Again, mouse hovering over champion icon)
Let's enjoy Riot's spaghetti code.