If a new csv is being created, it’s almost certainly because the logger is restarting. It could be some irregularity in the power that’s causing the board to restart directly or it’s the watchdog. If you’re not specifically setting the file name, whenever the board attempts to talk to the SD card, it puts together the logger ID and the date and checks if a file already exists with that name. If that file exists, it adds to it; if not, it creates it. So multiple restarts in the same day wouldn’t show up as multiple files, but you would see a new file for each new day there’s a restart. If the gap is 15 or 20 minutes, chances are really high the watchdog is firing.
It’s also possible that there’s something up with the connections between the SD card and the Mayfly, but since you’re seeing the issue on multiple loggers I doubt that’s it.