Celesteela was placed in A rank because of its ability to check a lot of threatening Pokemon in the tier. Mega Gardevoir, Mega Dianice, Alakazam and Jirachi can be checked by a Specially Defensive variant, while Pokemon like Terrakion, Salamence and Scolipede can be kept in check by a Physically Defensive variant that's been running around. Celesteela finds itself sitting in a comfortable place in the tier, and has been put in A rank.
Mega Sableye was also put in A rank because it's a wonderful Pokemon on not only stall builds, but also quite a few balance teams out there. The ability to invalidate nearly every hazard setter in the tier (Hippowdon, Skarmory, Swampert, Empoleon) is an amazing asset for any team, while also being able to check huge threats like Conkeldurr and Alakazam, as teams can easily lack switch ins for those two.
While the HDB Court Change set hasn't really been used yet, the CB set has already been a nice breaker for the tier. Cinderace's higher base speed than Infernape, albeit weaker, is huge for outspeeding Pokemon like Latias and Gengar, being able to sometimes break more sufficiently. It does struggle with some things, like being pretty one-dimensional, and failing to break through certain walls like Hippowdon, Alomomola and Swampert. Despite these drawbacks, Cinderace is sitting comfortably in B+.
Since Blacephalon left last shift, Chandelure is no longer outclassed and can go back to what it did before. SubCM is an amazing stallbreaker, which is a playstyle that's becoming increasingly more common. Choice Specs variants are also as hard to switch into as ever, with even Hydreigon taking. a ton from Specs Overheat.
With the tier looking for the ever so present Manaphy, Slowking has arisen as one of the better Manaphy checks in the tier. Its access to Dragon Tail allows it to phase out Manaphy after a Tail Glow boost, while resisting every hit that Manaphy wants to throw at it (unless Manaphy is running Energy Ball, which it never should). Besides this, Slowking also checks various other Pokemon like Mega Gardevoir, Keldeo and Primarina.
A --> A+
Manaphy never really caught on early generation, but ever since Thundurus' ban earlier this month, it's been popping off. +3 Manaphy is super hard to switch into, and Manaphy's coverage of Water/Ice/Psychic hits nearly everything in the tier outside of Empoleon. Its prominence on recent HO's and the destructive power it has on the tier allowed it to rise to A+.
A- --> A
Gengar rose because its Nasty Plot set is a phenomenal breaker in the tier. +2 Gengar, combined with Ghostium Z, is a menace to deal with if you're playing any sort of bulkier team. Its also been prominent on certain HO's, acting as a spinblocker.
A- --> A+
Rotom-Heat and Victini left, Scizor enjoys it. Even though Celesteela came back, it still has to worry about Knock Off. Scizor is back doing its daily business of SDBP, which is still threatening most teams out there.
A- --> A
Just like with Manaphy and Gengar, Xurkitree rose because people finally found out how good the setup sets actually are. +3 Electrium is a huge nuke for any team, and even Shuca Berry can be ran to take an EQ from the likes of Mega Aero or Terrak, and KO them in return.
B+ --> A-
Reuniclus rose to the A ranks because of the prominence of the Future Sight set. Because of Mega Venusaur's prominence in the tier, Reuniclus often finds a good opportunity to come on the battlefield in a match, using Future Sight, and dealing quite some damage if you're not a Dark-type, which don't always like to switch into Reuniclus because of the fear of Focus Blast.
A+ --> A
Amoonguss dropped because it's no longer as needed anymore as teams since Azumarill got banned. It's still solid, just less necessary overall.
A- --> B+
Altaria-Mega sucks in a metagame where Celesteela, Skarmory, Mega Venusaur and Amoonguss are all very good Pokemon, and you can't even beat most Dragons in the tier. Huge L for this mon.