It is part of the general cycle in MMOs that some classes are seen as more powerful than others (maybe better at their roles, easier to play, fewer disadvantages, more flexible, have access to some overpowered ability, etc). Over the life of the game, as balancing tweaks are made, the class on top will probably change.
Although people mock ‘flavour of the month’ players who switch from one powerful class to another, there’s no special reason not to do this if you have time spare to level and gear another class. Or rather, there’s no real in game reward for sticking with a class through thick and thin.
Hopefully if balancing goes well, there should never be a huge gap between classes. They should theoretically always be close enough that people can just play whichever they prefer without being disadvantaged. But for some people, even a small gap is too much when there’s no advantage to make up for it.
Do you think there are advantages to sticking with your favourite class when it is on a downswing? It will be rarer, for a start. People will know that you play it because you love it. Maybe those are social advantages. Or are you also more likely to get bitter and burned out, especially if your class then gets some love and all the flavour of the month players flock back?