Because he defects to the rebels. Possible named NPCs to take a stand:
Spoiler:- Magister Hathorel. He features in the 8.2 scenario where Honor-Horde and Alliance kill Sunreavers to spring out an apparent traitor. He's extremely hateful of Jaina and the Horde rebels in the scene.
- Rommath is another. He's always been resentful of Dalaran, but for Jaina his hatred is tenfold thanks to the Purge, which he witnessed first hand. He would feel the same as Hathorel for the events in 8.2. His closer relationship to Lorthemar would also make their feud more dramatic.
And once more, I don't think the presence of the Sunreavers at Baine's prison area was random, but a deliberate inclusion by Sylvanas. She expected someone to spring Baine out, but why have Sunreavers specifically ambush the perps? Also, if Nathanos is to be believed, she even planned for them to fail. Sunreavers, victims of Jaina's injustice couple of years ago, once more slain by her, with the assistance of Horde rebels. It can serve as Loyalist propaganda, but more specifically drive a wedge between blood elves and Lorthemar, who sides with the rebels despite such an event. Sylvanas apparently expected him to defect too.