Originally Posted by EatYerGreens
If it has happened to you consistently at 12 provinces but regardless of which faction you were, the leader's influence rating, or the geography, then disregard this next bit...
I'm speculating that it may have been to do with the geography, closely followed by a 'chain reaction' effect. Necrotherion only said 'Northern Italy' but, for the sake of argument, let's say the Emperor's stack invaded Rome and his other stack invaded The Papal States. Assume Naples is still held by A.N. Other faction.
All it would take now, is for the movement of troops from Genoa, Milan and Venice - to participate in these two invasions - for a sudden loyalty dip(*) in those three provinces. If they had all rebelled simultaneously, on that basis, THEN the Emperor is cut off from the rest of the empire and you get the 'chain reaction'.
It could also be the case that a province with a siege in progress isn't counted as a valid connection-path back to the remainder of the empire. So the Emperor could be besiegeing Rome, another army besieging Papal States (discounting it as a 'valid path') and it would only take a single rebellion in Tuscany to cut the emperor off and thus trigger the chain.
If I'm right then, provided you always ensure that there's at least two possible, valid, connection paths back to your domain, in case one of them rebels, then you should be able to fight battles, with your king in command, without this kind of collapse.