I wasn't sure if I accidentally altered something in the Creation Kit, so I looked at the Details tab of my mod. Along with the usual 'REFR' list of references and 'FACT' for added factions, there were a couple of 'D' markers for two instances of 'RELA'. What is 'RELA'? Should I hit 'ignore' to un-delete them?

Relationships? Between NPCS.

Bingo!

http://www.uesp.net/wiki/Tes5Mod:Mod_File_Format

Oh, OK. That must be a dirty edit. I'll undelete it - thanks.