what about ahab's habits was mysterious to the sailors on board the ship

From the text:

"With many other particulars concerning Ahab, always had it remained a mystery to some, why it was, that for a certain period, both before and after the sailing of the Pequod, he had hidden himself away with such Grand-Lama-like exclusiveness;

and, for that one interval, sought speechless refuge, as it were, among the marble senate of the dead."