The story behind my renegade guard is still evolving somewhat. Basically, they are the personal army of a nobleman on a very feudal world with a large number of nobles who are constantly struggling against each other for power. This particular nobleman has run afoul of the Imperial authorities because of his peculiar and heretical interest in alien cultures. He has been interacting and trading with Tau, Eldar, and other races of foul xenos, and he even uses some of them as mercenaries in his army.
Tau authorities have discovered that a few of their race have defected and joined this human's army, and they aren't too pleased about it. They have sent a small force to infiltrate the planet and capture or kill their wayward brethren. They discovered a squad of Tau mercenaries in an infantry platoon that was isolated patrolling some city ruins (for some irrelevant reason). Seeing their chance, they set an ambush for the patrolling Imperial Guardsmen.