This kind of depends on what the question refers to as a “sudden reappearance.” This can be interpreted two ways:

A champion suddenly becomes a powerful meta pick having had no direct changes. A champion suddenly appears in the patch notes after having not been seen for a while.

In the first case, the meta tends to naturally evolve as we change parts of the game every two weeks in standard patches. Whether it be runes, items, champs, or other systemic things like Baron or Rift Herald, every change can shift the meta in small (or large) ways that can benefit some champs more than others. If we removed Grevious Wounds from the game, any champion with high health regen would likely climb pretty high in terms of priority in the meta without any other direct changes. If we buffed Grevious Wounds, they’d probably fall substantially out of favor. That’s a pretty drastic example, but I hope it helps illustrate the point.

In the second case, some champions may appear in the patch notes suddenly due to a variety of reasons, but more often than not it’s because a designer has an idea for what they believe makes the champion better long-term for League—like the work done for Lux a few patches ago. Your perspective on League’s landscape can also affect whether a change to a champ feels random or not. If you care about pro play and you see a meta-dominant champion get nerfed in a patch, it probably makes sense to you. If you don’t watch or follow pro play, you might wonder why the hell we nerfed a champion that doesn’t seem to be considered powerful in your matches. The majority of our patch-to-patch work is prioritized pretty heavily (no, we don’t use a dartboard to choose our targets), but every now and then we work on things that may not immediately be recognized as important in the current meta you’re used to.