To the best of my knowledge, AG doesn't mention a timeline of Regulus and Polaris rebellions or the state of the political systems in place. The information about it being a quiet system are, in fact, from NTF forces sending you off to your eventual death.
There is no real mention of a catalytic event, something to spur "normal" people into rebellion. No mention of long standing animosities or regional differences that would be expected in a breakaway type of rebellion.
In my opinion (and that's all AG is, my opinion on the NTF), the rebellion was too serious for it to be a slow percolation of resentment in 3 systems. There is no mention of infighting within these systems that defected (not systems that were overrun), no mention of civilian unrest and no mention of massive parts of other fleets in other systems attempting (and obviously failing) rebellion.
Does that mean it didn't happen? No of course, it's just not stated. However I find it very hard to believe that the NTF staged a slow, growing rebellion among loyal GTVA fleets in 3 systems, stopped, and then began attacking other systems.
To me (again my opinion), making sure your systems were secure of loyal men before attacking neighboring systems seems to be the most effective means of starting and sustaining the NTF rebellion.
Every member of the GTVA in those systems wasn't in on the deal obviously (as reflected in AG), but I don't buy the idea that Bosch took his fleet rebel on the hope that down the line some other systems might join him. Also that the GTVA wouldn't strike back on the one system that rebelled.