Gathering pearls in a shell field carries risks.
The ocean is a vast treasure trove accessible to the powerful; humans are invaders, while the sea tribes are the masters. When their methods of benefit clash, countless disputes of various kinds arise, even leading to life-and-death struggles.
Over the years, during the harvesting seasons of the shell fields, such vicious incidents have been far from uncommon. In truth, it's hard to speak of right or wrong; why should you be entitled to harvest what is unclaimed and I not?
Human pirates wouldn't attack at this stage because they lack the ability to dive to the seabed and harvest pearls themselves—they'd exhaust themselves to death.
Pirates generally choose to strike when the harvest ends in January, waiting until the vessels are fully laden to make their move; on the contrary, the sea clans tend to create disturbances during the pearl harvesting. As for their motives, only heaven knows.