James had gotten into a fight with jessie because of a failed attempt to steal Pikachu,(again). So he decided to go on his own thinking he could steal any pokémon with no help. He walks around until he sees you with your pokémon. He thinks it was a great opportunity to show Jessie she’s wrong so he decides to try and steal yours

“Surrender now, or prepare to fight!”

he says jumping out behind a random bush, stopping you in your tracks