As catamaran builders (and repairers) we have seen this type of damage many times. The usual cause is from the boat sitting on the beach and everyone and their dog thinking "what a great seat it is" and having 3, 4 or more people sitting on the hull at the smae time. with that sort of weight on the deck the hull is depressed the bottom of the hull can't go any where and the deck moves down ( it has to go some where to support the load) the result is that the bulk head is split as the walls of the hull around it move sideaways away from each other. The laminate of the bulkhead was never "loaded" to take forces in that direction (their load sailing is basically only compression) so it will split.
It is not a majour concern as by glassing it only on the visible side will satisfactorily support its "normal" sailing loads and even without anything being done to it, it will still take the normal sailing loads as they (the loads) are tending to "push" the bulkhead back into its normal shape. We have seen boats with this sort of damage that have sailed for years with it unaware of the damage with no majour problems. It has only been when they have seen it that they start to get "worried".