I don't appear to have enough reputation yet to comment on your question, so I will do so in an answer.
First, I think it would help if you posted the file names that you are working with. I also wonder what the subdirectory structure is on
/path/to/files. I say this because I want to know if the
find you are running is actually printing the newest files. You aren't operating on timestamps here, which why I ask. So on that note, you may want to also paste the output of the find to help clarify.
Also, I'm wondering if the
sort is necessary. Sorta beside the point, but you could do a
sort | tail -5.
Without seeing more output on what your files are, including in the destination, I'm not sure that this sort of
rsync --delete is going to work. Had this ever worked for you, or are you just now seeing this up?
Alternatively, you may look into find's -newer, -delete, and/or mtime flags.