It's algorithm based, right? So if you post once, and all your regular fans show up, give likes etc, get you into popular. Then you update again. Maybe some fans assume it's the same update they just saw, maybe they only check the app once a day, maybe they know you'll update again later so they hold off--they don't come out as strong for the second one. So your views come from new people who are less likely to Heart, the ratio drops, you fall out of popular.
I don't have any greater insight into the algorithm than anyone else but it makes sense to me?