They had demoted Pluto on the basis of the discovery of Eris. They also discovered that Eris had a moon. It was thought to be much bigger than Pluto. That turned out not to be true. So having accepted Eris as a planet, they had to consider carefully what to do about Pluto. It has a gravitational constant of 0.82 m/s^2 which is 8.4% of that of our gravitational pull. and slightly more than Pluto.
Later data showed that these two objects in space were more like twins and that's what shook up the definition of a planet. You can't demote one and not the other.
