Not all the details are in, but apparently the captain began backing down on the huge fish, a common practice in big-game fishing when a fish is taking line. He puts the boat in reverse to chase the fish.
One commenter on Marlin Magazine's Facebook post who apparently had some knowledge of the incident said that the captain fell as he was backing down on the fish at full throttle. The boat took on too much water and, finally, there was no correcting the situation.
Marlin Magazine reported that the boat went to the bottom of the sea and everybody on board was rescued by the photo boat. And, of course, the fish got away.
More photos as the boat sinks and the fish continues to jump: