Well the main problem with Orks is that they release spores on death that mature to other orkoid species: squigs, snotlings, gretchin, and then finally orks, what's curious about the problem in Pragia is that no squigs, snotlings, or gretchin are being found in large numbers, but feral orks continue to harass major cities - including the capital.
How the orks got there was that a rok came there unintentionally as it was headed towards the fringes of the Octavian sub-sector but wound up in the Atellus sub-sector
PDF forces and reinforcing guardsmen regiment neutralized the main body of the orks, they are currently doing a "sweeping" of ork spore concentrations (although they will never be able to completely burn out the orks, like all planets invaded by the orks)
There is a few more oddities about the ork infestations as well, as they seem to be not coming in a trickle, but coming in waves and also targeting key infrastructure (which indicates strategy, which is very unorky)
the bulk is feral, but somehow there are a few flashgitz with heavy weapons, and even though it would indicate there is a "clever" ork warboss, there has been no sightings of the larger, browner, orks leading the feral and flashgitz
this task is also sort of a fool's quest for Metalus, as helping the mop up efforts of an ork invasion is considered to be a mundane chore for the likes of an inquisitor