Celebi rose to A- due to its ability to handle new omnipresent threats such as Mega Slowbro, Mega Diancie, and Mega Lopunny with its NastyPass set, and versatility, as it can run at least 4 differents sets effectively (NastyPass, offensive NP, Scarf, defensive utility)
Gyarados rose to A- because it pairs excellently with new physical MEvos on offensive teams, such as Mega Lopunny, Mega Metagross, Mega Altaria, which automatically makes it more viable. With physical offense becoming so potent, Gyarados is an excellent sweeper to pair your MEvos with, and it can also bypass Thundurus with Lum Berry.
Hippowdon rose to A- because of all the new physical threats it deals with, as well as being one of the best counters to Thundurus and Mega Diancie with a specially defensive set.
Rhyperior dropped to B because physical offense has become way too unfavorable to it, and every single new physical MEvo either 2HKOes it (Mega Metagross, Mega Lopunny) or sets up on it (Mega Altaria). Birdspam is also not such a huge focus of physical offense right now, due to the big amount of amazing physical sweepers and cleaners available, which means that Rhyperior is not keeping in check as much stuff as it used to anymore. Plus, Mega Sableye does its SR job harder, and Dragonite and Mega Tyranitar also dropped a lot in viability, two dangerous Pokemon that Rhyperior was a great check to.
Suicune dropped to B because Mega Slowbro is the preferred CM bulky Water-type on teams, and the metagame has also adapted to dealing with bulky boosting threats due to the introduction of Mega Slowbro, and the influx of Pokemon that make Suicune's life miserable, such as TG + Energy Ball Manaphy, NP Celebi, NP Thundurus, SubDD Mega Gyarados, DD Heal Bell Mega Altaria, etc.
Mega Swampert dropped to B because of heavy competition in the Swift Swim and MEvo department. As a Swift Swim user, Mega Swampert is a bit lacking in power and also needs one turn to MEvolve, which can be detrimental against a boosted foe. It does somewhat makes up for it with its bulk and great typing, but the fact that it's a MEvo puts it one rank lower than Kingdra and Kabutops, which provide better cleaning and sweeping potential and don't occupy your Mega slot.
Tornadus-T dropped because the metagame is more focused around physical hitter, which Tornadus-T can't switch into. Furthermore, the majority of the popular special attackers beat Tornadus-T, such as Greninja, Thundurus, Mega Gardevoir, Mega Slowbro, Mega Diancie, Mega Manectric, Mega Latias, and Mega Sableye. Finally, Torn-T received new checks in Mega Metagross and Mega Diancie, making it easier to check with offensive teams.
Victini dropped because Mega Altaria, Mega Diancie, and Mega Slowbro are new checks and counters, making Victini's job more difficult.
Alomomola dropped because set up sweepers being everywhere really hurts its viability, as no matter how prepared you are for them, Pokemon such as SD Mega Gallade and SubPass Mega Lopunny will just give you a hell of a time.
Amoonguss rose to B+ because it deals with some important new threats, namely Mega Altaria, Mega Diancie, and Mega Slowbro. Plus with Mega Venusaur getting harder to fit on teams due to the increased opportunity cost, Amoonguss enjoys the slightly decreased competition as Grass / Poison type. Although Celebi gives it big competition, Poison-typing, Spore, Clear Smog, and Regenerator give Amoonguss plenty of reasons to be used.
Mega Camerupt dropped to C+ because it's too slow, doesn't hit that hard, and takes up your MEvo slot. Many Pokemon can avoid the 2HKO from it while OHKOing back, such as Latios, Rotom-W, and AV Azumarill, while even more Pokemon can check it once and force it out, which is a big problem for a Pokemon as slow as Mega Camerupt, without any kind of recovery and not that impressive bulk.
Chesnaught rose to B+ because Drain Punch is amazing, providing Chesnaught with a reliable STAB move that can restore it to (almost) full health after KOing foes such as Bisharp, Mega Gyarados, Mega Lopunny, and Terrakion, plus Chesnaught deals with a few new or improved threats, such as Mega Gyarados and Mega Lopunny.
Lucario dropped to B- because the new MEvos make its life miserable, with Mega Diancie and Mega Metagross being great checks for offense, and Mega Slowbro being a hard counter for defensive or balance teams. Also, Mega Sableye and Mega Altaria force Lucario to run Iron Tail, which in turn leaves it vulnerable to Pokemon such as Gliscor, Landorus-T, and regular Slowbro.
Scolipede dropped to B- because full Baton Pass teams took a hit with the introduction of Mega Sableye, Mega Slowbro, and Mega Diancie. The two Magic Bounce users can't be Taunted, put to sleep, or phazed, and can start boosting to sweep the whole BP team, while Mega Slowbro can do the same if it has Oblivion in its base form and uses a RestTalk set.
Zapdos dropped to B- because flying spam is less popular and most top tier therats have no trouble getting past Zapdos, a Pokemon that is already Stealth Rock weak and doesn't have impressive bulk.