In the last Final Fantasy XIV patch, several balance changes came, including to the Crystalline Conflict mode that was added in patch 6.1, Newfound Adventure. On June 7th, Naoki Yoshida says the team will revert the recent changes following player feedback and an investigation into where the mistakes were made.
The balance changes were based on data from earlier this month and because it takes time to write the patch and schedule the changes, the balance adjustments weren't as relevant when they were implemented. The Black Mage, for instance as of Friday, May 6th, was showing the lowest usage and lowest win rates on all player categories.
When they consider PVP data and win rates, they take into account several categories including player job, whether a player is playing casual, ranked, or even expert, and other trends. Their goal is to keep win rates for each job hovering at about 50% and when it comes to balance changes their goal isn't simply to play with the numbers but to try to change things in a way that keeps the characteristics of the job and gives players a better chance at victory.
When it comes to the Black Mage, a spellcaster, this job's style relies on area of effect attacks and damage over time, and also limiting enemy movement. The recent changes, while intended to help increase use, viability, and win rate, made the Black Mage very overpowered. This change could have happened by mistake for a different job, it just fell upon the Black Mage this time but Yoshida says that the example created particular frustration for many players.
For more, see the post at Final Fantasy XIV.