That's normal. Note the preview list that's turned on at the bottom of the rename dialog in your screenshot. In order to update that Opus has to run your script to find out which filenames it will generate for the selected files.
If your rename script has side effects, and doesn't just pass back a new filename to Opus, then those side effects will be triggered.
I suppose rename scripts could be passed a flag telling them it's only a test, but rename scripts weren't really designed with side effects in mind.
You can avoid the problem, while still "abusing" rename scripts, by putting the script into a button instead of into a rename preset (like this). That reduces clutter in the rename dialog's preset list, too.
(I've added a note about this to the post about the technique.)