Does anyone think a cause of procrastination might be... the fear of Death? Being lost in thought and having to suddenly divert your attention to other things means you're erasing your current state of mind, never to return to this exact combination of thoughts and feelings again.
Things like browsing tumblr are often coupled with thinking of something important to me in the back of my mind. To put my own question in a less interesting way, I think this desire to capture the fleeting realization I seem to be on the verge of makes me delay quitting the mindless tumblr browsing and having to think of something else.