I guess they crossed paths while Morgana's immortal army was sweeping the lands. I'm pretty sure Percival's family died at the hands of her. He hates everything to do with Morgana, as said in A Servant of Two Masters. I believe his family died at the hands of her. Lancelot travelled the lands getting himself into much trouble (as seen in Lancelot and Guinevere), and he met many people- Percival just happened to be a very noble one, and they became good friends. It just happens, you know. ;) Besides, Percival had no one else and he had to use his skills for a good cause!