Pretty sure there is a large wad of SB's and Safety info published by Airbus on the issue. It has happened for many years and probably boils down to mainly finger trouble and lack of attention to detail. Poor old hook systems tend to be neglected and it is not limited to B3's.
Unintentional hook release doesn't get reported so much. How much the hook gets used and the value of what is being slung influences things. i.e. whether you're humping rocks and hay bales or a geophysical array.
Mis-rigged and incorrect manual release cable installation is an issue.
US Dept of the Interior has summed it up pretty well
here.