Every few months a single blog post will get 10x traffic for a day. Why?

I’ve been blogging for years and noticed a trend I can’t figure out. About every 3 months a blog post will get 10 times the normal traffic for a day or two. It’s been happening for about 1.5 years, about the time I started getting good organic traffic. It typically happens to a middle preforming post, ranking somewhere 5-10 getting about 50 visits a day. It returns to normal in a day or two then over the next month that post tends to climbs to a top 3 ranking.

The traffic doesn’t appear to be bots. It comes from all over, normal analytics and conversion rates. It doesn’t appear to be anything that happen for that day either, my niche is very evergreen. And it happens very consistently. Any ideas about these spikes?