Pluto has been designated a planet in our solar system for years (ever since it was discovered in the last century), but in 2006 it was demoted.
What caused this decision? And is there a chance that it could be reversed?
Edit: well, http://www.dailygalaxy.com/my_weblog/2017/03/nasas-new-horizon-astronomers-declare-pluto-is-a-planet-so-is-jupiters-ocean-moon-europa.html is interesting; this is science, so anything could (potentially) change.
Pluto is now classified as a dwarf planet. The main difference between a planet and a dwarf planet has to do with the requirement that a planet clear out the material in and near its orbit. Planets do this, dwarf planets do not.
The reclassification was triggered by the discovery of many additional object (the Edgeworth-Kuiper Belt) out beyond the orbit of Neptune. Some of the objects are nearly as big as (and is a few cases, possibly bigger than) Pluto and in very similar orbits. Thus it was realized that Pluto was just the largest of a large number of objects in the outer solar system.
This is simply science at work. At the local university, we have an Astronomy textbook from the 1800's that lists the 12 planets: Mercury, Venus, Earth, Mars, Ceres, Pallas, Juno, Vesta, Jupiter, Saturn, Uranus, and Neptune. However, as more objects were detected between Mars and Jupiter, it was realized this was a new class of object and the middle four were downgraded from planet status to asteroids. It is the same process at work today out in the outer solar system.
Pluto is still considered a dwarf planet. This was because it did not meet the full criteria for being classified as a planet. Most notably it did not clear its orbit of other debris.
This is still considered controversial as many scientist do not agree with the definition of what a planet is and still consider Pluto to meet planet criteria. So it is possible that this may change especially if the criteria for what is a planet changes due to new discoveries. This is very possible with the work the Kepler telescope is producing. As we discover new planets in large numbers you can be sure they will be finding ever different planets that will force a redefinition of what a planet is.
As many already said, Pluto is now considered a "dwarf planet"
For your second question, there is no chance that Pluto will be reclassified as a planet again.
Pluto has been reclassified as a dwarf planet.
It was reclassified as such because a growing number of objects were found similar to Pluto, which exhibited at least one notable difference from the other planets. The choice would have been to accept these other objects as planets or to develop a new class of object.
The primary features of a planet are that it has an orbital path 'clear' of other debris, orbits around the sun, and is massive enough to maintain hydrostatic equilibrium. Pluto (as well as other dwarf planets) fail to meet the first criterion.
If you are interested, there is an audio recording of the IAU General Assembly session on the definition of a planet http://www.jodcast.net/archive/200608IAU/
I don't really care what the IAU voted. Pluto will always be a planet in my book. Astronomy is full of historic inaccuracies that we perpetuate for tradition sake. Some examples that come to mind are early/late-type galaxies, Population I/II/III stars, and brown dwarfs.
Mercury, Venus, Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn and Uranus are the primary planets in the Solar System.