1)
Why did Edom rebel now?
1.
Radak: It was due to the sins of Yehoram ben Yehoshafat.
2)
Was it rebellion to appoint a king?
1.
Rashi: Yes. From the days of David, they did not have a king. "u'Melech Ein be'Edom Nitzav Melech" (Melachim I, 22:48). Malchei Yehudah appointed the Netziv, and he was like the Melech. He is even called Melech Edom above (3:9), when he joined with Yehoshafat and Yehoram ben Achav). Eight kings ruled in Edom before a king from Yisrael (Bereishis 36:31-43), and correspondingly there were eight kings in Yisrael, from Ish Boshes until Yehoram ben Yehoshafat, during which there was no king in Edom.
