(Or updating your social page status or tweeting.)
When there's nothing going on there's nothing to write about. (Duh, obvious.)
But when you're in the middle of a busy patch, in order to get something up, you tend to post in haste, often something poorly written (certainly not your best) and sometimes grouchy in content (because when you're over-busy you're annoyed at the world). Certainly not an in-depth discussion of whatever important project or event you're in the middle of; there's just not time to give that proper attention.
To make matters worse, because you're busy, you don't have time to put anything else up, so the lousy post hangs there for days, in prominent view at the top of the page.