In reply to rodw:
Okay, had a look and perhaps the way the bag is sitting on your back is knocking the buckle on the lid straps open?
I just passed the extra loop of tape over the buckle then into a larksfoot which should prevent that from happening.
If you think that there is a fault, please bring it into a Tiso shop and we'll have a look ... I'm in the Perth shop but am sure any of my colleagues would be happy to help.
Bruce