just got done playing a game as 
and toward the end of the game i noticed that my attacks were dealing almost 500 damage to the inhibitor towers, nexus towers and the nexus when our 
didn't deal nearly as much with an auto. i was not casting skills at this time. i'm not sure what the cause was, or how to recreate it.
i had around 830 ap, we had 3 dragons and i did not have the mage elixir. my build was 
The way that the damage calculation to turrets goes is you get Base attack damage + the greater of either Bonus Attack Damage or 40% of your AP. Then if you have two Dragon stacks that damage is increased by an additional 15%
So Ahri has 104 Base AD at level 18 + 332 (40% of your AP) + 65.4 (15% from Dragon Stacks) = 501.4 damage to towers on hit. That gets reduced by the 100 Armor the turrets have naturally, which puts you about where you were saying for damage.
ok, i thought i was tripping balls on this one, thanks for the 
guys.
