I need an explanation

Im playing Bard at bot lane. It is 12 minutes into the game. Enemy Lux is being chased by my Riven, but he has not hit her in a bit. Lux is going to suicide against out t2 tower at bot. I see this, and I try to lock her down with my ult so my team can get the kill. Turret shoots first shot. She has one more before she is dead. My ult hits my t2 turret as the second shot is about to hit. She gets hit, and I get the kill.

You can check the replay on my summoner highlights.

Any explanations?

TY