Irelia Q resetting when it shouldn't! [GAMEPLAY]

Hi, so I was playing this game of Irelia in ranked and this happens... (Only happened ones, but obviously not intended to be there) So I'm Q'ing the minions to last hit as usual, but when I Q the canon right after one of the minions, it still hasn't gone on cooldown.

I can't make any step-by-step instructions on how to reproduce this, but I could imagine it has something with amount of stacks and the speed that it's happening in. :)

-Xensational