No doubt hook and elastic is going to be the most fixable but I have experienced similar panniers jumping their top hooks off the top rail of the rack on rough ground.
I've also experienced problems with the elastic/hook at the bottom. Unless you have a horizontal bar to hook onto it's virtually useless and that horizontal bar ideally needs something at either end to stop the hook slipping off.
No doubt hook and elastic is going to be the most fixable but I have experienced similar panniers jumping their top hooks off the top rail of the rack on rough ground.
I've also experienced problems with the elastic/hook at the bottom. Unless you have a horizontal bar to hook onto it's virtually useless and that horizontal bar ideally needs something at either end to stop the hook slipping off.