Nah, I rather think that the AI goes "Hey what is going on here... Ok I better get in a position where I can formulate a good strategy (hence sieges broken off) and I need to secure myself (hence acceptance of Protectorates)."
Basically the AI goes into 'start'-mode. It starts over again as if the game starts at that date, which seems to be a fitting explaination as the info to make the AI realize it was actually in a running game could possibly be rather substantial, and an obvious place to cut if size was demanded smaller.