You don't have a lot of friends do you.
The meta changes based of nerfs/buffs to items, champs, auxiliary (towers/dragons/jungle). As well as game play innovation. (4 man tower pushes, double TP, ect.) A champion that is over-tuned is is OP. That's what the phrase means.... A champion that is under-tuned can not be OP. There is no blanket statement for why all champs are OP or UP. Each one is different.
Riot takes a careful approach to nerf/buffs. So it can takes many patches before a "problem" champion that players (and/or pros) are not happy with. Tahm is a perfect example. Riot **admits ** they were shocked at the frustration players had with Tahm. The fact that Riot views him first as a support and he was rocking over 60% ban rate for a while really caught them off guard.
So having more bans gives players a way to deal with champions that are OP while Riot looks at the issue. Rengar/Tahm/Gangplank/Ryze/Fiora/Graves/Mundo were all thought of has really strong to the point that players didn't want to deal with them in the past few patches. And we see some trickling of nerfs on them.
As far as why champs appear to just show up out of nowhere and become pick/ban worthy in pro play; it's innovation and number/kit changes. Once Faker showed how to button mash corrrectly on Ryze, pros followed suite and saw how crazy OP he was. Mundo with the item changes (reduction of the strength of last whisper, increase of the power of tank masteries, ect.) he became OP, and the pros saw this.
If you want I can walk you through more issues. Just try not to be so confrontational.