Why do certain moves/ Pokemon get banned from one tier? And how are bans implemented?
Chatter's power was changed from 60 to 65. From this generation onward, sound can no longer be recorded. Instead, the move will always confuse the target. Despite no longer being able to record sound, Chatter still cannot be copied by Sketch.
That gives reason for the ban. 100% chance to confuse is annoying, and makes it overpowered in the PU, which is a notch below NU.In other words, the Pokemon of NU are better at handling such tactics that are guaranteed to cause confusion. Or, rather, the strength of the NU Pokemon can counter the slight disadvantage provided by the Confusion, which the Pokemon of PU can not, because they're simply not good enough to bounce back from such a disadvantage.
As to how it is banned, well, it takes place in the following steps:
First off, someone puts up a post in the forum for PU, about why they think something is overpowered. Should it be decided that it is a valid claim, then it reopened for testing in a separate thread. Then, the Smogon Community has to decide, by playing time and again on the Suspect Test phase, whether a move / Pokemon is really overpowered or not, and then a voting is held. There's an elaborate (slightly) hierarchy involving Tier Council Members, and a rotating Council for specific Suspect Testing. Finally, based on a simple majority (50% +1), it is decided to ban (or not) based on the results. A better form of democracy than in most countries, if you ask me, but that's probably because of the far lesser numbers and obviously, way less important decisions, but whatever.
After the ban, the PS! server is reset, so as to prevent players from being able to use specific movesets, such as Chatot with Chatter, in this case, while searching for PU battles. This takes some time to be implemented, due to obvious reasons.