1)
Why does it say that Mo'av rebelled?
1.
Radak: Mo'av used to serve Achav, like it says below (3:4) "v'Heshiv l'Melech Yisrael Me'ah Elef Karim." After Achav died, Mo'av rebelled against his son.
2)
Why does the Sefer begin with Mo'av's rebellion?
1.
Malbim: It teaches how Hashem began to punish Beis Achav. (a) Mo'av used to serve Achav; after he died, Mo'av rebelled. (b) Achazyahu fell in his Aliyah (verse 2), a place not proper to fall. Even so, he did not repent.
