My 2009 mac mini randomly reboots. It is running OS X El Capitan 10.11.6

If your Mac spontaneously restarts or displays a message that it restarted or shut down because of a problem, click here --> https://support.apple.com/HT200553

Are you using a new peripheral? Something to do with MSDOS?

MSDOS is the precursor to Windows. You can still see it associated with using FAT formatted flash drives which often come with a Windows default format. MSDOS was indicated as the last loaded kext before the crash. I was wondering if you had some old thing on there that was invoking the kext that provided a service for DOS format.

Kernel Panics can have either software or hardware causes. It is usually easy to spot a software one, particularly if a kext load points to an old or badly coded piece of software. Hardware ones are tougher to spot because you just have to associate them with the computer doing something when it crashes.

You can try running your computer in Safe Mode and see if it stops it. On the other hand, running in safe mode is very restrictive and not fun after a minute or two.

I recently had a set of KPs on my computer and I eventually tracked them down to a particular piece of software i would leave running. I stopped running it except when absolutely necessary and the issue cleared up. You could see if you can associate yours with something that way.