I've found a few of reasons why they can fly open. The plastic nut that holds the lock in tends to loosen off allowing the catch to disengage, so I put a smear off hot glue on the edge of the thread and the nut. The second of my 22lt's had a bent wire locking catch that wasn't engaging the hook on the lock properly. The third reason is not closing the lid properly, it can look like it's closed but it is not latched properly. I now make sure I push down on the lid after closing it.
Regards Martin.