I think this is the answer, from the campaign script

monitor_event FactionTurnStart FactionType norway
and not I_HotseatEnabled
and CampaignDifficulty > easy
and not IsFactionAIControlled
and I_NumberOfSettlements norway >= 30

set_counter AI_attack_player_level 2

end_monitor

The best way to combat this would be by getting small, useless cities to rebel so that you don't hit the magic number of 30 until you are down to one AI opponent.