Mojo Darkale went against the Pandaren's who had allied with the alliance in order fight the blood elves. Most pandarens located in Alliance lands considered blood elves and naga to be dangerous and evil, and chose to ally with the alliance against them. These pandarens tried to keep blood elves from getting to outland, and later tried to stop them from reaching Northrend. However Mojo Darkale did not share the opinion of his brethren and chose to join Prince Kael and Lady Vasj to outland when they broke out of the dungeons and escaped through a portal to outland to escape the Alliance.
Notes
If you find the secret sheep area in the Blood Elf Campaign during the mission where you have to help the Blood Elves escape Dalaran Prison, you will unlock a secret Tower Defence stage where you have to build unique tower to prevent Garithos' men from destroying the portal to outland.
If you win that stage, you will be given a Pandaren Brewmaster for last few levels of the blood elf compaign. IN the first level he appears in, you can only control your heroes and are suppose to escort Illidan (trapped in Maiev's wagon) back to your base.
Having Mojo Darkale in this stage makes things a lot easier.