I play the danish. (that sounds funny) Same thing happened in my game. I'm sure the invasions are scripted or at least factions invade specific territories in a specific order.

Often I'm allied with a faction and on amiable terms when they decide to attack me.

Back to those belligerent Scotts. I was allied to them and feeding them cash every turn by the thousands in order to create turmoil in England. I was also strong in the North of Europe with any number of easy targets as neighbors. They had three territories on the island and had the military to take the island. They landed north of Brughes and wait for a few turns. I thought "Good luck lads, Caen is that way. Here's military access." They finally attacked my city of Brughes with too little force to even garrison if they succeeded at taking it.

So these guys deserve to die. I'll chop their arms off and toss them in the North Sea for christianity!