Kidnapping
Control, control, control, it was all about control. Rodolphus hadn't meant for the result of his spell to be so large but he wasn't exactly in charge of all of his faculties at that particular moment. One thing was certain though, even to him, and that was that the pair needed to be separated if Bellatrix was going to get what she wanted. There was no way he was leaving before they'd gotten what they had come for.
Thankfully splitting the lovebirds shouldn't be too difficult and he sent a banishing charm after James to tear him away from her. Even if he chose to deflect it, Rodolphus didn't doubt Bellatrix would use his distraction as an opportunity to force Lily's hand. He could make it impossible for James to protect both himself and his house mate.