Here’s a possibility. Say a popular camera comes with a particular size of memory card which can store 531 typical images. You might fill the card, clean it out, then take one picture for uploading tho the web. That’d be 532.
Or, perhaps the numbers aren’t incremental – maybe it takes a hash of the date and uses that, and the firmware default date is one that comes up with that number. However if that were the case, you’d expect the curve to bump up at that point and continue decaying from there, not return to the original curve.
My Olympus cameras (all three I’ve had) don’t start off at 0, they make a hash of the date then append a counter to that, I think.
So for olympus users you would see a big spike at (say) P20111225001.jpg or whatever it works out as if people get given them for christmas.
However given the isolated anomoly on 532 my money’s on a characteristic/bug of a popular camera firmware.