Vayne attack speed confusion.

I have recently started playing vayne adc. I was just going through all the vayne's abilities and the descriptions. I realized that vayne does not have any ability that lets her stack attack speed. At least, i couldn't find anything in the ability descriptions. But during the vayne play, many times i noticed that sometimes when she is autoing, her attack speed goes up all of a sudden. Usually, when you stop attacking, it comes down again. Is that some item effect? Can't be her abilities cuz it didn't say anywhere that she does that. Is that BORK?