All my backups just... disappeared?

I know the summary might sound a bit alarm-triggering.
But I cannot explain yet why all my backups of over a year have just disappeared from my external drive.
I don't discard this is a case of PEBKAC ("problem exists between keyboard and chair")

I'm on Debian Unstable running backintime-qt4 1.1.12
Today, after seeing that the LED of my external HDD was still blinking since more than 24 hours, I checked what was going on. I saw that the BIT icon was there on the systray, and a quick glance showed me that a full backup was taking place.
I wondered why a full backup was being run. I still wonder.
When I went to the backup folder to check what could be going on, I found it almost empty. There was only a backup from 3 days ago. Any other backup from previous days and weeks and months were just lost.

These are some things that I recall doing that might be related to my backups disappearing:
- a few weeks ago, I changed the method of backup to "rsync". But, at that time, after letting BIT run a few times with the new configuration, I've checked and all my backups were still there.
- a few days ago I run GSmartControl (the GUI for smartmontools) to run some checks on the external HDD. There is a chance that I've run it while a backup was being run. Could this have caused problems and/or be related?

I'm not going mad because, well, I don't think I've missed anything and now BIT is backing up my folders again. But I can't figure out what could have happened.

Thanks.