To give in some lore from Vermintide 1 (and overall):
Clan Fester, more specifically their Breeders are suffering under the Brood Blight, a disease which makes the Rat Mothers (super old lore, WHFRPG 2nd edition) only give of weak, if even living offspring, which means that the numbers of Clan Fester are finite.
They were once on the Council of Thirteen, but were cast out of it (up until the Black Plague, around 1111-1143, also from WHFRPG 2nd edition). They eventually became a sub-clan of Pestilens.
During V1 Clan Fester was solely under the control of Grey Seer Rasknitt, who promised them a cure to the Brood Blight, which even Clan Pestilens had no interest in dealing with after learning all they could about it, fearing to infect their own Breeders with the dangerous illness, and shunned Fester after they contracted it.
After the almost complete destruction of Übersreik, the pact with the Rotbloods and the construction of the Skittergate, Clan Pestilens was most likely intrigued by their rise in power (and ignoring the Blight 
).
Skarrik Spinemangler, a pawn by Clan Pestilens (info about that could be gained through Olesya through her after-mission dialogue, now no longer accessible 
), was set as the new chieftain for Clan Fester. The previous chieftain, Krench, was killed (by the Ü5) shortly after making the pact with the Rotbloods in Stromdorf. Barbeque Honeymustard was most likely his liasion, as he get’s mentioned later to be the culprit in stealing Lohners books during Death on the Reik.
So, they were only recently getting under the renewed control of Pestilens, thus the addition of the Plague Monks in V2. The addition of the other new skaven units was most likely happening under their influence as well as the higher status Rasknitt got for making the pact; we don’t know if he was responsible for that, Krench was a mightier chieftain than Skarrik, he had a Warpstone amulet which could shield him from harm, an immensely potent and pricey trinket, so he might had the idea (it was also during the time Rasknitt was in hiding after his supposed defeat).
The disruption of the Waystone network (Athel Yenlui) was also done by Clan Pestilens, which would hint at a strong Plaguepriest/lord directing the Clan through Skarrik while still being occluded from the Ü5. In that he certainly did more to stop the group than Bödvarr did.
