I got myself a Givi tank bag so I can carry my camera gear with me, initially I was really impressed until yesterday when it decided to disconnect itself, luckily I was doing about 20 mph at the time and it stayed on the tank due to the safety strap.
At first I thought I hadn't connected it properly but I managed to make it fail again, it seems if the bag is mostly empty with weight in the end of it towards the seat, in this instance my disk lock it puts pressure on the mechanism and can make it come off.
I'm hoping it's just a dodgy bag I've got, I'm going to send it back for a replacement but it might be worth having a look if anyone else uses the tanklock system.