It's a coincidence that Tryndamere got buffed before his epic skin and nerfed a couple of weeks before its release. The same happened for Vlad, he was unplayed for months and everyone cried for buffs, he got buffed and received an epic skins, just to get nerfed a couple of weeks afterwards. You can also add Galio to the mix...
Other examples are Shen, Malphite, Orianna, Urgot, Jax and Sona.
The only clear exception to the previous examples is Eve, must likely because they are currently reworking her model and didn't want to create a skin for her hold model.
Riot doesn't release a skin as soon it is made. They can hold that skin back until a champ gets popular. Riot also has the power to make champs popular by buffing them.