Have you ever left somewhere, gone somewhere else, and forgotten why you went?
For example: leave your living room, go to the kitchen. But why? I'm holding a cup... did I just want to wash it, or was I going to refill it?
I just realized I have an equivalent computer-based behaviour: leave an application, start another, and forget why I opened it. It's especially problematic with a Web browser, because I could have been going to do almost anything! Might as well check that social network.