Today, marks the kick-off of the Midsummer Fires holiday. With it comes a change to the way the the holiday bosses work. Now, you directly queue to fight the boss by using the, 'Dungeon Finder' button and then selecting the boss's name. The vanity items (mounts, pets, and flavor) no longer drop directly from the boss, rather, they drop from a bag you receive the first time you kill the boss per day.
This makes it difficult to farm the rare stuff but easy to get the upgrades that drop from a given boss.