It seems to happen with boosts sometimes. When they take a while to pick up - for algo reasons - they seem to be given an extra lift towards the end of the 15-day period to make sure they get the 500+ views as expected. The same happened with my emigration piece. Quite flat, then a sudden spike to 500+ views, and now back down to normal.

I think maybe it happens if there are too many boosted pieces in a category/pub, so there isn't room for them all in the algo, and some have to play catch-up?

All stuff greatly appreciated - the pub's going well so far. I'll be doing a retrospective on the month and the boosts early next week, then a new month starts on Wednesday and the counter gets reset.