To get tenure in STEM, you need to publish 1 to 3 papers a year. If you only publish once per year, you'd be on the low end, so you always have to think about your next paper. You always have to try to work on something that would be publishable within the next 6 months to a year, but preferably within 6 months.

presumably in between thinking they're teaching classes, advising grad students, and publishing something -- because it's "publish or perish"