Are women aware of what their boobs are touching?

I know the title sounds completely ridiculous but it's happened twice now at work where a female coworker will come up behind me while I'm sitting down and she will be looking over my shoulder at something on my computer and her boobs are brushing up against the back of my neck in what I can only call a firm manner. It's the same coworker both times. Could this be accidental or does she know what she's doing?