Just strange that it started appearing a few weeks ago. Well after installing
Mavericks and before making the 10.9.1 update. Not strange enough for me to bother investigating, just enough to pester you!
Just for more grins, I deleted the plist and manually ran SD. It did
not reappear in that case. I suppose it is created/moved/copied by the code/script that runs when the backup is a scheduled event.
Oh well, take the rest of the day off and rest up for a Merry Christmas! Hope you have a great one with friends and family! See you next year!