Vayne question

I just have a question (I don't care why if I sound like a noob here), but why do most high tier adc vayne players tumble on the second activation of their w? They sometimes use their tumble in different times, but when it is up, they seem to tumble on the second part of w. I would think that tumbling on the third activation is better because the damage stacks and you are in range for more comboes. But then again, maybe they do it on the second part because they want to deal consistent damage over time and in teamfights they have it up a bit sooner (even if vayne's q is on a 2 second cd by then). Please help me.