Self replicating worm in sd card
Check for suspicious applications you may have installed recently. Also this behaviour is very weird as it should usually do that on the internal sd card where it could harm your device and not on the external one where it only annoys the user.

About the "self replicating": If you know in what file the worm is located and checked if all files are the same then deleting that file and formatting your sd card may be the solution to your issue.