They were most likely running away from oryx while the rest of the cabal empire was busy fighting them, or they were sent to Mars to figure out the black garden and vex secrets. The former is more likely, as they are ill equipped to fight the Vex located on Mars. However, while running from the Hive, they stumbled upon the Vex and the black garden. At the time, humanity was still on Mars, hence the hostile take over. Remember: the cabal blow up moons just for getting in their way. They are extremely aggressive and militant, which means diplomacy isn't an option, even when facing their enemy.