The first logo was introduced at the 2014 San Diego Comic Con where a mood theme teaser was unveiled. Then at the 2014 BlizzCon, there was a promo with two websites (FightfortheHorde and FightfortheAlliance) showing the new logo; and after the release date was moved from March 2016 to June 2016 there is a brand-new logo also available in the press kit.
The first logo was criticized by some fans because of its similarity to the StarCraft logo. The current logo is more reminiscent of the original Warcraft: Orcs and Humans logo; and probably more welcomed and recognized by the fans.
The evolution of the logo is perplexing. Will there be a new logo after SDCC 2015 or will this be the final one? Only time will tell.