Actually, it's not as much a mystery as it sounds. The timeline indicates the likelihood of poor judgment or being caught unawares by a storm. By the time they arrived at Esperanza Inlet, after leaving from Gold River and going through Muchalat Inlet and up Tahsis Inlet, very nast storms were either brewing or imminent.
They were due back within days and there is a strong possibility they either decided to risk it, or got caught unawares on the way south. Either way, the storms were so bad by that weekend that even the fishing boats weren't going out. One scenario is they got south of Esperanza when they realized they were in trouble, found no where they could land and tried to run back to the inlet.
They were well equipped, had at last two years experience kayaking in the area but something went wrong.
Did they fail to read the signs of impending poor weather? Was it overconfidence as they tried to outrun the storm? I suspect we will never know exactly. But the fact remains they made a decision which left them caught in a storm in an area known for its shipwrecks.
Lesson: read the weather and delay if necessary. Better to be late, even miss a day or work, than not get back at all.