The Etiquette of Proportional Warfare
Over the last thousand years an unnofficial protocol has developed in times of conflict to keep down escalation and prevent border skirmishes from turning into interplanetary war. The protocol is imperfect, and it is sometimes flaunted by the arrogant, the angry, or the ambitious, but on the whole it has sufficed to keep most conflicts down to a localized level and preserve the continual cold war between Houses without them turning into the Emperor Wars more than once every five hundred years or so.
The etiquette of proportional warfare is based fundamentally on the concept of peerage, the idea that all nobles are on some level equal. It is in many ways an expansion of the same ethic that drives duels - conflicts between peers to determine who is the better man. Stated most simply, it is conventionally assumed that conflict occurs between peers and that it is impolite to interfere.
Thus, if a baron (and given the at least ideal relationship between a noble and his lands, this means a barony) attacks another baron, the conflict is considered a matter for barons, and is left for the two of them to resolve. If instead a count attacks a baron, it is assumed that the count is in fact attacking someone of his own rank - and so the victim baron's comital liege or allied countess will respond to that attack as if she had been so challenged. In effect, this both prevents escalation - a web of alliances being triggered by a single attack - and also keeps the strongest of nobles from preying without fear of reprisal on their weaker brethren.
The etiquette usually but not always goes the other way - which is to say that if a baron attacks a count, it is generally assumed that the baron is a chess piece being moved by the hand of someone unseen with comital dignity. Consequently, it is considered more than appropriate to respond to that baronial aggression with the full resources of a county, leaving the baron in very poor shape indeed if he is fact acting alone rather than serving in the van of a larger campaign.
This informal protocol is enforced through social and political sanction. When a greater lord interferes in the affairs of a lesser lord in defiance of this protocol, he is in effect doing one of two things: either treating the lesser lord as if he were an equal (dangerous indeed if that lesser noble is a rising political rival) or demeaning himself to the level of his lesser (costing him respect in the eyes of his peers.) The reprecussions of this in the political scene can be quite substantial, as those who might have bargained with him before no longer chose to do so or those who would not treat in the past with his enemy now do so freely, following the example of the 'respect' the greater lord has shown his lesser enemy.