Balancing changes in relationship with revenue
This is yet only a hypothesis, and I want to elaborate my point. Please correct me if I am wrong at any part, or provide me with counter examples. And if you agree with me, don't judge Riots: they are running a business, and my opinion does not serve to berate their practice; it is just for knowledge. Many other game communities are aware of how the companies practice their business, and how their game play is affected by revenue (ex: World of Tanks, World of Warships, War Thunder, Smite, ...)
I've always suspected that Riot balancing changes have something to do with skin revenue, and not just for the sake of gameplay balance alone. If they can buff the champion to increase its play rate, and therefore skin purchase rate, without too much negative comments from the community, they will do it. A buff does not necessarily make huge gameplay changes, but think of it as a way to grab players' attention to the champion. They may buff the champion prior to, the same time as, or after releasing new skin; as long as the play rate and revenue are affected.
Looking back, Ezreal got a 2 huge buffs in 6.17 and 7.17, the same patches where Arcade Ezreal and Star Guardian Ezreal were released. And they kept on buffing him, and then leaving him in that state for a few patches, only to nerf him heavily, or reverting those changes later. The Ezreal we end up with today is pretty much the same Ezreal we had pre 6.17, except for some global changes to maskmen.
Another shining example is Lux. As we know, Lux has had a flurry of new, expensive skins, to the point that the community got sicken of them. In the same manner, Lux was buffed to the sky, and there is no sign of reverting back. Correct me if I'm wrong, but I think that it was because they released too many Lux skins close to each other, each is expensive, so those skins' compete each other's revenue, and ultimately result in lower total revenue. And that is why they still have to buff Lux once in a while, since Riots haven't met the expected revenue out of those skins yet. I also suspect that when they do reach that revenue, or they find that keep buffing her is not effective anymore, they will quickly nerf her to the ground in a few patches (or in a big seasonal change).
More recently, let's look at patch 8.17, 8.18, and 8.19. In 8.17, Talon, Irelia, Thresh, Urgot, and Lucian got new skins of 1350 RP and more. In that very same patch, Irelia's Q got buffed. Talno got a rather indirect buff in the form of Duskblade's buff. Lucian was buffed heavily in the next patch, and indirectly buffed via nerfing Storm Razor, thereby nerfing poking/ quick bursting ADC that normally counter Lucian like Draven, Trist, Vayne, or Caitlyn. Thresh also got buffed recently, and arguably much more heavily than other, since it fixes his long CD in laning phase. Such is to be expected, since Thresh also has an incoming chroma in 8.19. 8.18 sees the release of Odyssey skins, but strangely without any buff to the champions with new skins. However, I think that it is because most of those champions' play rate is independent of buffs anyway: those champions have not received any direct changes recently, yet have lots of one trick players; they will play those champions regardless of buff or nerf. In patch 8.19, apart from Thresh's buff relating to skin, Kha'zix also received an abnormally heavy buff. Coincidentally, Kha'zix also has a Championship skin and 25 chromas coming out. Other champions with Championship skins did not receive buff, which I think is due to the masteries of those champions: players maining those champions will spend a lot of time playing them regardless of buff or nerf. I even think that, to begin with, those champions qualified to receive championship skins are champions that take lots of experience to master, and will guarantee constant play rate regardless of patches.