Legends say the first leage meme was "better nerf irelia"
Even if a champion remains static in their kit numbers, items and other reworks can change the state of the game enough to make them oppressive. This is sometimes referred to as flavor of the month, where a champion has had nothing special happen to it but suddenly becomes popular. In some cases, this is because they are so rarely seen that no one knows how to deal with them (galio) rather than being particularly overpowered. In other cases, the champion has had some light touchups (swain) coupled with item changes that turn them into a monster.
In swain's case, his ult went from sending out 3 crows to 5, each of which heal him for a straight amount, rather than a % of the damage done (reduced numbers for healing off minions). This is basically a straight buff to his team fight survivability. His other changes were nice, but not super impactful in terms of bringing him to the fore. This combined with item changes (rod, tear, SV, etc) has made him a bit tankier than before. In addition, only two items apply grievous wounds, so it is rather hard to counter his healing early in lane.
Irelia benefited a lot from the tri force changes and other small item changes over season 6. The mastery changes have also helped her out a bunch.