The whole course of events was actually very simple.
After the Roger Pirates reached the final island—Laugh Tale—Roger used his ability to hear the Voice of All Things to speak with Joy Boy's soul and learned from him the truth about what had happened eight hundred years ago.
Combined with the many things they had encountered during their voyages over the years, such as the atrocities committed by the Celestial Dragons at God Valley...
Roger began to entertain the idea of rebelling against the World Government.
But after learning from Joy Boy about the World Government's true strength back then, they knew that the Roger Pirates alone clearly could not stand against it.
So Roger, already terminally ill, voluntarily turned himself in to the World Government and used his own execution as the price to usher in the Great Pirate Era.
His goal was to throw the entire sea into chaos and weaken the World Government's influence.
As for the other members of the Roger Pirates, they were all scattered across the world, ready to help the "prophesied king" when necessary.
This so-called "prophesied king" was Nika's inheritor.
Before Joy Boy died eight hundred years ago, he had made a prophecy about this: that the Nika Fruit would awaken again eight hundred years later.
Thus, he made an agreement with many races and nations that had received his aid, telling them to wait for this "prophesied king" to lead them down the path to freedom.
It was just that the Roger Pirates had reached Laugh Tale too early; the day of the prophecy was still twenty or thirty years away!
So Roger told Shanks about these matters—at the time, Shanks had not entered Laugh Tale because he was taking care of the sick Buggy.
As a mere teenage boy, Shanks