system
Given the following post, respond as if you're commenting on Reddit:
human
When Pluto was downgraded to a dwarf planet what was the cause of this? ie. more accurate observations? a recalculation of its size? A change in the definitions of planetary size?
gpt
The cause of the "downgrade" was the discovery of multiple bodies that are now called "Trans-Neptunian Objects" (TNOs). These TNOs are of a size comparable to Pluto (but a bit smaller) and orbit the Sun in similar orbits.

Before these discoveries, it was assumed that Pluto was the only significant object in its orbital region, just like it is for the 8 planets. But the discovery of the TNOs showed that Pluto is different from the 8 planets in that it has a lot of compatriots in its orbital region.

This brought up the question of how many planets there are exactly. If Pluto is a planet, then one could say that several of the TNOs could be classified as a planet as well. So instead of facing the possibility of having to deal with a greatly increased number of things we call "planet", the decision was made to be more strict in calling something a planet. 

In 2006, the International Astronomical Union came up with 3 requirements for something to be called a "planet" in our solar system:

- It's in orbit around the Sun.

- It's big enough to have reached hydrostatic equilibrium. In less fancy terms: It's (mostly) round.

- It's big enough to have cleared the neighborhood around its orbit.

That last requirement is the one that Pluto fails. It's not the only thing of importance in the area around its orbit. Along with the new definition of a planet, the definition of a "dwarf planet" was introduced, which is anything that meets the first two requirements, but not the third, and is not a satellite (moon).

This definition covers Pluto and several other TNOs.

So the short answer to your question is that the discovery of multiple objects that are comparable in size to Pluto led the IAU to redefine what it means to be a planet and this new definition dropped Pluto from the list of planets.