Pluto should not have been classified as a planet in the first place when it was discovered back in 1930 by then 24 year old Clyde Tombaugh. Pluto fails to meet multiple traditional planet criteria: it is the smallest by far, it had a perihelion that intersected another planet's orbit, its eccentricity is the highest, and its ecliptic inclination is also much larger than any other planet.
To be fair, at the time of its discovery, astronomers thought that it was rather larger and more dense than it really is. Also, it was not until the past couple of decades that the nature of the Kupier Belt was understood; now we know that there may be dozens of Pluto-like objects scattered on the fringes of the solar system. Indeed, it was perhaps in the mid 1980s that there first began a movement by some astronomers to remove Pluto from the planet list. I think that they were unsuccessful for a long time in part because Tombaugh, who by all reports was a really nice and humble person, was still alive and many astronomers thought it would be an insult to him to "degrade" his discovery. (He passed on in 1997.)
Pluto is not the first body to get bumped. The asteroid Ceres, some 600 Km in diameter, was called a planet for a while in the 19th century before the nature of the asteroid belt was better understood.