Quote Originally Posted by EatYerGreens
In one Byz campaign, my trade income was ticking along at a modest level and other factions tolerated the presence of my ships. Just ONE turn after sticking a single extra ship into the Baltic, adding about 3 or 4 extra ports onto my trade portfilio, the Sicilians chose that moment to attack a ship, declaring war and blockading vitually all of my trade income, as their ships were in just about every sea zone that mine were.

That's just to illustrate the point that, sometimes, the AI isn't so dumb. Even when it's watch towers do not overlook your provinces and it's agents aren't in town, their ships off your coasts CAN detect the sudden jumps in revenue which the stepwise expansion of your trade links can cause. When it doesn't share a land border with you, with which to start the war, the sea battles are the only method it has to hinder your progress. Hence the expression 'Total War'.
That might be an explanation to. We do know that the A.I. is programmed to be a nuisance even to the point of suicide, and disrupting or preventing sea trade is at best a major nuisance. Also, this kind of naval aggresion doesn't start until your trade networks reaches foreign coast, which seems to support your idea.

On the other hand, the delayed naval aggresion can also be caused by coastal defense routines of the naval A.I. (perhaps assassins that are not on foreign missions work like this?). Also, suicidal programming doesn't kick in until you reach the 60% mark, so either argument works both ways.