I really can't get this. With such a strong feudalism, does kneeling down mean so much? I think there is not much difference between being 'King in the North' or 'Warden of the North'. The difference is only symbolic. Why did the North rebel when they are practically independent and why did the İron Throne go to war for territories that they don't actually dominate? I don't know if any tax regimes or any foreign threats for Westeros exist.
The actual catalyst for the rebellion was the imprisoning of Ned Stark for the crime of conspiring to hijack the Iron Throne. It was this event that lead to Robb calling his banners. Ned Stark was not only the Warden of the North, thus the overlord of all the Northern lands north of the Neck, he was very popular. His vassals adored him, even idolized him. Not only was he a Stark, he was also a fair, honorable, and just overlord who also had the reputation of being a war hero who avenged his father, helped destroy the Targaryen dynasty, and put the Baratheon dynasty in it's place. In many Northern lands he is known simply as "The Ned". Thus the North refused to believe that it's hero could conspire to steal the throne of his best friend. Thus the rebellion to free Ned Stark.
It wasn't until Ned's death did the thoughts of independence creep in. Not only were the Northeners angry and clamoring for vengeance, they had had a bellyfull of Southron politics and power plays interfering in their lives. The Northeners had long complained that the Southrons didn't understand the North and it's problems, and just wanted to rule it. The Greatjon summed it up in his rousing speech that led to the King of the North movement:
MY LORDS! Here is what I say to these two kings! Renly Baratheon is nothing to me, nor Stannis neither. Why should they rule over me and mine, from some flowery seat in Highgarden or Dorne? What do they know of the Wall or the wolfswood or the barrows of the First Men? Even their gods are wrong. The Others take the Lannisters too, I’ve had a bellyful of them. Why shouldn’t we rule ourselves again? It was the dragons we married, and the dragons are all dead! There sits the only king I mean to bow my knee to, m’lords. The King in the North!
The motivation for Northern independence goes much deeper than Eddard's death.
Eddard Stark was the third member of his family in less than twenty years to be executed by the King on the Iron Throne. Eddard's older brother Brandon and father Rickard were both killed by Aerys the Mad King.
A generation earlier, the North supported Robert Baratheon's rebellion after the deaths of Brandon and Rickard. They decided to place a more acceptable ruler on the Iron Throne while leaving the general relationship between North and South unchanged.
After the death of Eddard, the North could have taken the same course again by replacing Joffrey with Stannis or Renly; but what would be the point? Once more, Northerners would fight and die to determine which southern lord sat on the Iron Throne. In the next generation, there might be another mad king, disputed succession or civil war, and the North would be drawn in yet again. (Important lords such as Umber and Karstark were old enough to remember Robert's rebellion, and this would certainly have influenced their thinking.)
In the long term, there is little benefit to taking sides in the struggle for the Iron Throne. There are other reasons why independence is an attractive option for the North:
- It is culturally distinct from the South, particularly in its religion.
- It originally submitted to the Iron Throne out of fear of the Targaryen dragons, which is no longer a consideration.
- It is very easy to defend from a southern invasion, as any invading force has to pass through the marshlands defended by Moat Cailin.
- The wildlings beyond the Wall are a significant strategic threat to the North; wars in the South are an unnecessary distraction from possible wildling invasion.
There are also immediate, personal reasons to support independence. Eddard was very popular, and his bannermen are angry at his murder; and Robb is an attractive leader, especially after his victories in battle.
So for all these reasons, the northern lords proclaimed Robb as King of the North.
The North rebelled against the Iron Throne because Joffrey killed Ned Stark. Maybe before that there wasn't much difference between King and Warden of the North, and that's why the Starks bent their knee to the Targaryens so many years ago and never rebelled against them after that.
But when Robb found out that the King had captured his father (the Warden of the North) and planned to execute him -- and later actually did -- he is furious and he gathers his banners together for the rebellion.
The Rebellion was not for the title; the rebellion was for the fact that the King murdered the Warden of the North.
Also, the North (Winterfell etc) does belong to the Iron Throne. After Aegon the Conqueror conquered Westeros (~300 years before the show), he united the whole land (Westeros) into one realm. Everything there belongs to the Iron Throne. There hasn't been any change since then, so the North is dominated by the Iron Throne.
The Northern Rebellion's key failing was that they were unclear on whether they were after independence for the North, or vengeance for the death of Eddard Stark.
If the goal was Northern independence, they should have put a garrison at Moat Cailin and started building fleets to guard the coasts, then withdrawn back home. Those defenses would have been more than enough to hold the North against attempts to bring it back under the jurisdiction of King's Landing, especially with winter coming; and it would have allowed the northerners to get in a last harvest, and thus be better able to survive the coming winter.
If the goal was vengeance for Ned Stark, they should have foregone declaring independence, to better facilitate an alliance with one of the Baratheon brothers. As for which one, I would suggest Stannis, since he would be more likely than Renly, if he won, to insist on punishing anyone who didn't declare for him immediately.
But instead, they tried to have it both ways, seeking independence while also seeking vengeance. Making themselves unacceptable as an ally to Stannis, while simultaneously not making the necessary arrangements to make their newfound independence stick.