In every season you have one or two fotm classes, although their spec usually changes over time.
And it's a combination of overpowered damage and/or toughness through newly introduced or changed items.Just a quick glance at the twisted sword mechanics with the Delsere set where a multiplicator in the thousands gets multiplicated with another one in the same region will tell you how imbalanced this is. I don't think it slipped through, I assume they make those changes intentionally to boost a certain class for a season.
But especially with the LoN set and the possible variations it brings it might be a game changer for a class if someone found a new combination (that was unnoticed before or because there is some bug that tweaks the numbers).
The items/class/ability gets nerfed for the next season, and we have other classes rolling.
No other class right now known can do more dps and still have as high toughness than a wizzard. This will not only help people strifing for the top ladder, but it will also help people who have problems progress or farm a lvl 75 rift. It's not required, but it will make things easier.