Oberyn was going to launch his own rebellion? what did I miss?

“His father ignored the sally. “Prince Oberyn’s presence here is unfortunate. His brother is a cautious man, a reasoned man, subtle, deliberate, even indolent to a degree. He is a man who weighs the consequences of every word and every action. But Oberyn has always been half-mad.”
“Is it true he tried to raise Dorne for Viserys?”
“No one speaks of it, but yes. Ravens flew and riders rode, with what secret messages I never knew. Jon Arryn sailed to Sunspear to return Prince Lewyn’s bones, sat down with Prince Doran, and ended all the talk of war. But Robert never went to Dorne thereafter, and Prince Oberyn seldom left it.” (Tyrion VI, ASOS)