RESOLVE: At maximum Flow, the next instance of damage coming from a** champion or monster** against Yasuo will first convert his Flow into an absorption shield. After 1 second, if not already depleted, the shield dissipates and Yasuo loses all his Flow.
So apparently the shield only get's activated on champion or monster hit, not turret hit.
Basically if you didn't pop his shield, Yasuo would have walked under turret, took a shot, the shield wouldn't have activated (or done anything to negate damage), Yasuo would have died to the turret and been executed anyways. Same is true if he died to lane minions.
Yasuo would have died to the turret regardless of whether you popped his shield or not since the turret doesn't even activate the shield.
Popping the shield did nothing to contribute to his death. Unless you actually touched his health bar or inhibited him through cc, then you technically didn't do anything to help kill him.
Now if he did the same thing but died to a jungle monster, then there's a problem (since jungle camps do proc his shield, and the jungle camps would have had to eaten through the shield to kill him, which if you pop the shield means they don't have to eat through the shield,) but otherwise it's working as intended.