In gorkamorka etc orks have been stated as very hard to eradicate as their bodies sporulate and eventually adult orcs will emerge again from the ground.
My best guess would be for orcs/orks to have several reproductive cycles/methods. sexual through adult biped vertebrate greenskins and a base level fungal reproductive cycle. given the orc tendency for total annihilation warfare and the oldone/zoat warorganism theories, such a fallback option is only logical.
Just to drag up ork sex again; would it be too much to assume that Orcs and Orks do not function the same way? Sure it's GW's modus operandi to copy/paste as much as possible, but it's not a stretch to think the Orks=Spores thing was invented for the sci-fi amenable setting of 40K. Sure, the Rogue Trader books mention stuff like Ork families, but they're more like clans, with no mention of female Orks and several early hints of the sporing answer.
Bloodbowl of course, in addition to being on the Fantasy side, exists purely for shits and giggles, which an Orcish cheerleader supplies just fine. Maybe they're just moobs. Or the orky equivalent of moobs. And while I'm here-
Astartes Verminus
Have you seen my gob? It appears to have been smacked off.