I would guess the cradle collapsed, dumping her on her starboard side, with the cradle parts putting some holes in her. She, and the cradle parts slid off the freighter, impacting the sea on her side. The bow may be torn open too, allowing the ingress of water that had her stern high in the water as she righted. I would guess she was otherwise fairly well sealed for the trip, with all ports and passages closed to the outside. Thus, she stayed afloat. It had to be a jaw dropping moment.
Although it appears she originally settled bow high in the water. Hmmm. Maybe there are just the few holes in her.