With the Imperium's resources stretched to it's limit another swarm has been allowed to build in the jungle which required extermination, this time the Order of the Fallen Angel was in a position to respond. With an entirely mobile force* the Adepta Sororitas raced to confront the threat before it had time to make it to the closest settlement, or worse, build further.
*he knows I hate trying to kill Rhinos
Fratribusia was never meant to be held. It was always a delaying point while the machine of the Imperium ground into place at strategically vital planets.