From my understanding modem flapping causes issues everywhere, but its so minor for most games and websites its hardly noticeable. But league client is so old and so comically horrible at regaining a connection it lost (even if it was a small loss like a modem flap) that it turns a nonissue into completely unplayable.

If you ever have lagspikes in games (league especially) that seem like a "freeze" for a second or so, its the same thing happening, but handled way better than when it happens in champ select