Cinderace's tier is constantly changing because the metagame is always changing.
Every time tiers have been established this gen, more DLC and Pokemon are releasing and returning. This upsets the tiers because some broken Pokemon are back/introduced.
Pokemon are usually tiered depending on how well they can handle threats and threaten others. If the Pokemon Cinderace can effectively check fall out of it's tier, or a Pokemon comes along that can do Cinderace's job but better, then it's tier will change. Or, maybe, it can check even more Pokemon and and do an even better job. Simply, everything has been changing a lot due to DLC releases.