The origins of the Magistracy Cavaliers lie with a "national guard" battalion (Clayborne's Commandos) that had been abandoned on Thraxa and gone native. Realizing that the Magistracy didn't care about their defense, the Thraxans seduced them into becoming an official security force for their business concerns. Ignored by the MCC, the Thraxans were able to increase the unit's size to two battalions.
When the Pirates War finally exploded in 3057, the situation came to the MCC's attention. While the MCC was going to simply disband the unit, by force if necessary, Magestrix Emma Centrella brokered a deal that allowed the Commandos to become a full line regiment with the agreement that a battalion be permanently stationed on Thraxa full-time.
During the Victoria War, Naomi Centrella stripped the defenses of the Magistracy in order to show her neighbors how far her military had grown. @p? As a result many Canopian worlds were ripe for the plucking especially Thraxa and Vixen. When they raised a stink Naomi and the Magistracy abandoned them entirely.
So what the hell happened that the Cavaliers would so easily abandon their homeworld? (Naomi was simply stupid, completely stripping the Magistracy of its defenders is definitely not the way to show how much the MAF has improved.)