What is the meaning of "va'Yiskasher"?
Radak: Rebellion is called Kesher (A knot), for the rebels are tied to each other with agreement on the same goal.
Why does it mention that [beforehand] Yehoram was guarding in Ramos Gil'ad?
Radak: This explains why the rebellion was solid. Had Yehoram stayed there, the officers would have feared him and would not have agreed with Yehu to rebel. Yehoram was guarding the city against Chaza'el, but he was stricken and went to Yizre'el to recover (verse 15); this is when Yehu was anointed and the officers banded with him.
Why does it mention that all of Yisrael were guarding the city against Chaza'el?
Malbim: This is why it was a good time to rebel; all of Yisrael were distracted with the war.
