I think I have an idea, but it could be wrong.
My understanding is that rankings are often based on the user-behavior — the more interest that’s shown from behavior (eg “time spent on page” or “click to more details”) the better the ranking is.
Your posts sent lots of traffic, but that doesn’t mean it was quality traffic. Some of those visitors likely had less interest than your typical visitors do. This could have caused some people to land on the page and exhibit behavior that would decrease your rank (eg “land on the page and bounce right away”).
Again, just a thought, but I think you need more data to understand the decrease in rank.