Laura J. Solomon and Esther D. Rothblum (1984) conducted a study to see if procrastination was about more than merely poor study habits. Apparently this was a common belief among psychology researchers in the 80s. They figured there were plenty of possible reasons to procrastinate that hadn't been studied: "evaluation anxiety, difficulty in making decisions, rebellion against control, lack of assertion, fear of the consequences of success, perceived aversiveness of the task, and overly perfectionistic standards about competency" (503). In the end, they determined that major reasons for procrastination other than poor study habits/time management boiled down to fear of failure and task aversiveness. (You may recognize these terms from Onwuegbizie and Collins's article cited in my last post. Solomon and Rothblum designed the questionnaires Onwuegbizie and Collins used.)
This is really important because, as Solomon and Rothblum point out, if time management isn't the major cause, we might not want to act like it's the only solution. If our students are procrastinating because they're afraid we'll grade them harshly, maybe the more helpful thing to do would be to help them be more comfortable being evaluated. Review drafts without grading them. Avoid making ad hominem comments on papers (the dreaded "I expected more from you"). Don't put an undue amount of emphasis on a single assignment (break it up into mini-assignments if necessary). If our students are procrastinating because they don't like the actual act of writing, help get them comfortable. Lots of in-class writing. Talk to students about their ideas before having them turn things in. Have them talk to each other. Have them write informally. Use mini-assignments that can later be used as jumping off points for or even sections of larger papers.
As for which factor accounted for most of the procrastination, the answer is statistically complicated. A large group of students reported task aversiveness, but the small group of students who reported fear of failure often reported that as their only reason for procrastination. So both angles are important to tackle.
Solomon, Laura J., and Esther D. Rothblum. "Academic Procrastination: Frequency and Cognitive-Behavioral Correlates." Journal of Counseling Psychology 31.4 (1984): 503-509.