Maybe the new King/Queen endorses an entirely new deity and plans on making the rest of the nation worship it as well? Fucking with peoples traditions is a sure way to get them riled up. The sovereign could also have issued a taxation of the people that some find lawful and some don't (like with King John in England). Maybe there's always been tensions between two or three groups in the population, and now that the sovereign has allied with one of them the others are rebelling?