seeing as the default search is not most popular and not newest I think it is just a natural occurance that some old photos rise to the top once in a while.
I noticed that one old photo of mine all of a sudden, after being silent for months, started selling 3x a day. Perhaps it was because of halloween (it was a pic of an old house) or perhaps because one or a few people needed it, and then it rose a bit in the 'most popular' search and then more people are all of a sudden finding it.