Here’s how Pokemon can drop from a tier. I know that for OU at least, the usage threshold is ~5%, meaning if a Pokemon is used on less than ~5% of all teams on the ladder, then it drops to the lower tier. This number might be different for other tiers, but it should be about the same. As for how Pokemon rise in tiers, it is a bit more complicated. Each tier has a tiering council that decides whether or not a Pokemon is healthy for that tier’s metagame. If a Pokemon newly introduced into the meta is clearly overpowered beyond a doubt, the council will likely decide to quickban it. Otherwise, they may decide to wait and let the meta settle and develop to see how things turn out. If they decide that the Pokemon in question is still a little overcentralizing, they might decide to start a suspect test in which there is a period of time when any person can gain a certain designated amount of points on the ladder in order to vote on the fate of the Pokemon in question. At that point, if the majority of people that achieved the point requirements vote “Ban” as opposed to “No Ban,” then the Pokemon rises to the tier above it.