Possible, as in Schrodinger's cat may have slept here. Unlikely. First, no corrosion (rust) is visible, so it wasn't weakened. Second, you mentioned the bag had been mounted on another bike; if it were a case of infant mortality (i.e. the bolt was flawed at manufacture) I'd expect it would have broken previously. Occam's Razor-inspired conclusion is that the bolt unscrewed, probably due to vibrations while riding. That's why I suspect it wasn't screwed in tight enough; that's what torque values on bolts prevent if applied during assembly. And a thread-locker might hold a bolt on that wasn't screwed in tight enough, but it's best to torque the bolt during assembly.