Bug#849382: [apt] Every package on the system gets silently upgraded to backports. The result is severe system breakage, malfunctioning and data loss.

[Raphael Geissert]

Hi,

As discussed via IRC, this could be a case of
https://bugs.debian.org/838920 in unattended-upgrades.

Cheers,
-- 
Raphael Geissert - Debian Developer
www.debian.org - get.debian.net