Bran Stark came to Kings Landing with his father and sisters as he was never crippled. After his father's death, Arya fled and after Joffrey's death, Sansa fled. Now Bran was the lone wolf among treacherous lions. And those lions in order to strengthen their power both over Dorne and the North and also to save themselves from Dorne's wrath, sold what they thought was the only remaining Stark to the Red Viper. Oberyn Martell had wanted revenge for so long. And now he had the chance to exact his revenge on both - the Starks for being the reason of his sister's suffering when she was alive, and the Lannisters for being the reason of her death. They will all suffer the way his sister did, regardless of their innocence or guilt.