Okay since some people have been wanting explanations on a few of these changes, I'm going to post them here. Feel free to respond if you disagree with a change or my reasoning, that's what the thread is for.
Roselia rose to S rank because of many traits that make it a defnining Pokemon of the PU metagame. First and foremost, it fits on and does well on almost any team you put it on thanks to its ability to run both offensive and defensive sets very effectively, its great typing that lets it check threats like Poliwrath, Simipour to an extent, Tangela, Raichu, and slower Rock and Ground types like Golem and Probopass, and most importantly its ability to set Spikes or Toxic Spikes. PU is a very hazard-centric metagame right now, and Roselia is a huge part of that. As previously stated, Roselia fits well on almost any team, meaning that just about any team can make use of Spikes or Toxic Spikes, which is further augmented by the fact that PU's hazard removers have all seen a pretty heavy decline in both popularity and effectiveness lately, not to mention the fact that they're quite difficult to fit on a team. Roselia also has a few other very useful tools going for it, including the amazing utility of Sleep Powder, its good power and offensive typing, and Natural Cure. There isn't one particular thing about Roselia that makes it S rank worthy, but everything it has adds up to make it a very metagame defining Pokemon.
Barbaracle moved up to S rank as well because of the fact that if it sets up and the opposing team is worn down a small amount, it's almost guaranteed to sweep any team lacking the rare and otherwise relatively inferior Scarf Simipour or Raichu (the former of which fails to beat Jolly Barbaracle). This wouldn't normally be that big of a deal, but Barbaracle's deceptive bulk with White Herb (which it can easily afford to use over a boosting item as unlike Carracosta it does not rely on the weak Aqua Jet to sweep), ability to force out a lot of Pokemon with its good coverage and power, and the fact that one of the best Pokemon in the tier can use Memento let it set up on a surprising amount of the tier. The following calcs illustrate Barbaracle's bulk:
252+ Atk Pawniard Sucker Punch vs. 0 HP / 4 Def Barbaracle: 96-114 (33.6 - 40%) -- guaranteed 3HKO (Knock Off does less because White Herb gets used up)
252+ SpA Poliwrath Vacuum Wave vs. 0 HP / 0 SpD Barbaracle: 110-132 (38.5 - 46.3%) -- guaranteed 3HKO
252+ Atk Life Orb Carracosta Aqua Jet vs. 0 HP / 4 Def Barbaracle: 74-87 (25.9 - 30.5%) -- guaranteed 4HKO
252 Atk Life Orb Rapidash Wild Charge vs. 0 HP / 4 Def Barbaracle: 190-224 (66.6 - 78.5%) -- guaranteed 2HKO
With Memento support, it can easily set up on anything lacking a super effective move (and even quite a few things that do, such as this: -2 252 Atk Ursaring Close Combat vs. 0 HP / 4 Def Barbaracle: 116-138 (40.7 - 48.4%) -- guaranteed 3HKO), and one it does set up it's incredibly difficult to stop, making it an incredibly effective Pokemon in the metagame and one that is worthy of S rank.
Ninetales is moving down because of the recent increase in popularity of faster Pokemon such as Rapidash, Jumpluff, Zebstrika, Simipour, and Floatzel. This means that Ninetales is hard-pressed to sweep most offensive teams, as does the fact that it has few setup opportunities in the current metagame. It's still a great Pokemon, but it's no longer quite as metagame defining it once was.
Leafeon dropped because the metagame is very prepared for it in general. Like with Ninetales, a lot of prominent threats outspeed it, and it has few setup opportunities outside of forcing things out. In general, Leafeon is hard pressed to sweep at all outside of sun in this metagame, much more so than Ninetales, which is the big reason why it dropped.
Stunfisk rose because it is a fantastic check to Zebstrika and Raichu, both of which are very hard to effectively prepare for otherwise, as well as being an excellent spreader of paralysis that is especially useful in such an offensive metagame. Most of the Pokemon that are immune to paralysis do not want to switch into Stunfisk as well, since they tend to be either walled by it or hit hard by Earth Power, further increasing its effectiveness at spreading paralysis.
Most of the rest of the changes were made because we (galbia, anty, and I) thought the rankings were getting a bit inflated, so we moved a lot of stuff down a rank in order to be more accurate with the rankings as a whole and avoid ranking inflation.
Anyway, I apologize for making sudden changes like this without providing explanations, but from now on I'll try to explain at least the important changes, and remember that as always you can ask me why any particular change happened or post in disagreement to it.