My theory. Xebec was an awakened user of the Gura gura no mi.
As Sengoku said, WB had the power to destroy the world. This was likely due to his fruit.
When Rocks saw what the CDs had done to Shakky, he became so enraged that he decided to sink the world. This is what caused Garp to ally with Roger to defend the CDs.
Had they not done that, the world would have been destroyed.