So why are there two PDF factions at war in the first place?
Because of the three players in the group and the similarity between the toy soldier forces of two of them (Webb and Gary).
Here's my write-up for why this is happening.
Amblecotius is an agri-world and exports vast quantities of foodstuff, fibres, bio-fuel (the Dark Millennium equivalent of bio-diesel), and related byproducts. This is worth a huge amount of cash, credit and prestige for the ruling House Huriatat who own a monopoly on imports/exports for the entire system as is granted by their Imperial Charter. Their single biggest customer is the hiveworld of Loschtzitay in the Novah-Tippen star cluster, a world that would be at risk of famine and wide-spread revolt should the continual traffic of freight haulers cease.
Many other houses jealously covet this monopoly. Furthermore it's an open secret that if the system changed hands yet the flow of exports and upholding of the Pax Imperium continued this would be regarded as a something of a fait accompli and of little interest to the far-distant authorities in the Emperor's Palace on Terra. This sort of thing happens all the time. The Imperium is rarely able to interfere in such forced changes of hands and is pragmatic enough to realise that in many cases of this sort of vendetta and private war where no overt threat to the Imperium exists it's business as usual after the firing and/or assassination has all died down a bit.
For such an agricultural backwater, Amblecotius actually has a disproportionately large number of men under arms in it's purely defensive army, the Planetary Defense Force. This large size is in the interests of securing the flow of trade and traffic, the occasional problems with Eldar piracy and slave raiders, and hostile local fauna. It also helps to ward off the threat of aggressive moves by other noble and mercantile houses.
Military service is compulsory for all able-bodied males, and amongst a section of the female populace - the latter drawn by lot making women a sizable minority within the PDF. Equipment for the PDF is paid for from the public purse (it is basically the private army of the Huriatat clan) and, as is usual for this sort of military outfit, is based around that of the Imperial Army but done "on the cheap" to represent the lighter duties to which they are accustomed. In the Imperium the Army is the force of the offence - the PDF is the force of the defence until the big boys can arrive.
Battalion 48, Amblecotius PDF pictured after another succesful xenocide operation. Xeno village codename G/78 had been found guilty in absentia of stealing the Emperor's firewood and illegally watering from His watercourses.
However despite conscription, the small population of Amblecotius Prime is not sufficient enough to support a standing army of the size required. Therefore a good third of it's strength is drawn from volunteers from neighbouring systems and conscripts from Huriatat feudal holdings elsewhere. This one third of the strength is a mixed bag containing everything from bored farmboys from other farm worlds to hiveworld scum, mercenaries, deserters, demobbed and invalid personnel from other PDFs and disbanded Imperial Army regiments, slaves, and penal troops.
There has long been something of a split between the native and non-native parts of the forces of House Huriatat, not helped by individual units being formed from one faction or the other. The native units have a stake in the wider society, the non-natives are viewed as mercenaries and brigands out for all they can get. Linguistic, racial and sectarian differences all play a role in this mutual distrust.
Recently House Enhausen has started a scheme several decades in the planning. Alongside their "official" religion of the worshipping the Emperor as a God, the Amblecotians revere two lesser agricultural deities, Eranu and Uvavu believed by the Inquisition to be corrupted folk memories of two early settlers of the system back in the Dark Ages of Technology. It is thought that this religion is harmless and possibly of benefit to a population bound to rural servitude and thus tolerated. It is tradition amongst worshippers that the names of both deities are to be said in as low and rumbling a pitch of voice as the worshipper can manage. It is thought that this attracts the good favour of both deities and failing to observe this attracts bad luck. This behaviour is widely mocked by the non-native PDF troops to whom it causes much amusement.
Inquisitor Geenunt believes this to be the oldest existing holoimage of early Hierophants of the Eranu/Uvavu twin cult.
Enahusen's agent provocateurs amongst the native units of the PDF (such agent provocateurs use brightly colour underclothing as a secret sign with which to recognise each other) have started a propaganda campaign claiming that the non-native units are a fifth column intended to wipe out worship of Eranu and Uvavu. This appears to many to be plausible - Amblecotius's past is littered with episodes where this religion has held to be heresy and the Inquisition were not always as tolerant of this in the past as they appear to be now. The ruling House Huriatat are themselves foreign imports and cannot be trusted to protect the native religious sentiment.
Meanwhile agents have spread different propaganda amongst the non-native units. In these version of events, the cults of Eranu and Uvavu plan to remove outsiders from the system through violence and genocide, and then proscribe worship of the Master of Mankind. Belief has been deliberately spread in this faction that if forced to chose between the two factions, Huriatat will simply sacrifice the lesser (them) in order to keep the greater (the others) onside.
With both factions believing that the other is about to strike and feeling unable to call upon the authorities for aid, open civil war amongst the PDF was laughably simple to kick off by provoking incidents where units from both sides are garrisoned.
The second stage of the plan was for House Enhausen troops and mercenaries to invade The Daughter, Huriatat's orbital palace and seat of government. The PDF would be disarmed, the Huriatat family put in front of a kangaroo court on jumped up charges of wilful neglect and incompetence and the system folded up into Enhausen's feudal holdings. A two year programme of temporary price drops in system exports would be used to butter up trading partners and help diffuse any misplaced loyalty to the deposed Huritats, while the imposition of a slave labour economy would dramatically slash costs and thus raise profits.
But the Enhausen planners miscalculated badly. Their agents failed to see that the difficult-to-predict Blood Drinkers would race to the aim of House Huriatat, instead believing that no Space Marine Captain would hold that anarchy amongst serf soldiers on a farm world was worthy of their intervention.
Enhausen has now created a massive disruption to vital interstellar trade and cannot see any easy way of benefiting from it unless the Marines somehow fail in their campaign or decide that events elsewhere are of more importance.
Plans to remove the Marines from the sphere of influence are hurriedly being drawn up.
TL;DR - Native and non-native elements of the PDF set at each others throats by House Enhausen who aim to take possession of wealthy and destabilised system. Unfortunately Space Marines got involved, PDF fought back in self-defense. House Huriatat, the supposed rulers are left impotent and wringing their hands while trapped in their orbital palace.