time machine automatically added my external hd to exclusion list

Has this happened to anyone?

Today I happened to open Time Machine and look at the exclusion list to find that my portable external hard drive, which I carry with my macbook everywhere and is where I store most of my files, and which I have always included in the backups, was now listed as being excluded. Entering Time Machine, I see that the last backup was in February of this year so it must have been excluded immediately after that. But I can't think of why the drive would have been added onto the exclusion list. I haven't changed the settings for at least a year. I have two drives that I backup to: one USB and one NAS, alternating. I've had this set up for about a year. I did move recently though, and I think maybe it was around February that I set up my NAS at this new place and resumed backing up to it along with my USB drive. I know TM excludes external drives by default. Is it possible that TM somehow reset itself when it reconnected with the NAS, which is why the external was re-excluded?

Of course I took it off the list again, but I'm just trying to understand why this happened, so I can avoid anything similar happening again in the future. It was pretty scary to discover, because I have very important documents on that drive that need to stay backed up.

Thanks!