The possibility of missed opportunity seems to be a huge motivator for people in general, but most observably for me in writers. Despite accepting submissions year round, the three days before the deadline on both of the magazines I work on have more volume of submissions than any three weeks the rest of the year.
Such violent fear is easily circumvented by planning, but it's easier to plan the writing than what comes after. That's why Write 1 Sub 1 seemed like such a fantastic thing when I heard about it in January. Deadlines for magazines are less of an issue when you have personal deadlines at much more frequent intervals.