Why was Rechav'am punished shortly after he sinned? Yaravam was not punished so quickly!
Malbim: Hashem promised David "[your son] will be to Me like a son, that when he acts crookedly, I will rebuke him with the staff of people, and My Chesed will not depart from him" (Shmuel II, 7:14). When he veers from the good path, I will not leave him until he is corrupted until [he requires] destruction. Rather, I will immediately punish him to return him to the good. Rechav'am sinned in his fourth year, and was punished in his fifth. Hashem did not do so with Yaravam; he did not punish him until his measure [of sin] was full, and a decree of destruction was finalized on him.
Why does it say "Ma'alu", and not "Chat'u"?
Malbim: Hashem did not wait until sins increased. Rather, immediately after they transgressed, he rebuked them. Then Rechav'am saw that the masses that he gathered from the 10 tribes did not help him.
