Or, better (though less generalizable until you've answered it several times), 'why did you procrastinate on this particular occasion?' The thing that I'm procrastinating on is only the reason for the procrastination sometimes, at least for me. It's more common for the procrastination to be caused by the presence of a compelling distraction, or by trying to do the work without the right kind of lead-up or preparation (which is not reliably a function of the type of work; it's more closely related to other aspects of my life).

Asking the right questions is important, and the most common ones aren't reliably right.