Could be any number of reasons and somewhat pointless to speculate,as only they can answer you. 

You could wait to see if you drop leagues, or maybe don't try as hard to grow, see if that helps.

It sounds very much like a glitch.  Send a ticket to plarium which you can do via "Menu" then "Ask a Question".

Had your Kingdom been able to enter the recent Fury events and somehow placed first in those giving you the opporunity to work tier 9 tiles and if your three members worked incredibly hard at that maybe you could have risen so rapidly. But those conditions seem unlikely to be met and it is barely conceivable even then with the limited number of troops and limited number of marches available to you.