Ork reproduction may be much more complex than has been detailed so far. Many organisms have multiple stages to their reproduction with often very different physiologies and lifestyles for each one. A lot of these are to facilitate either moving from one environment to another or achieving sexual reproduction.
All Orks may be permanently sexually crossbreeding with other Orks, just not in the direct manner in which mammals do it. Instead spores land and grow on a suitable site, then when more spores from another Ork land on them, they absorb them, genetically mix with them, then produce a new crop of spores formed from this mixture. These are dispersed again. This process of mixture and dispersal goes on constantly, spreading and mixing Orkish genetic material. A specialised form for the purposes of mating, which may be totally unlike the active and growing form, is actually quite common in nature.
If these Orkoid fungal growths become large enough they start giving rise to squigs, which make conditions better, which allows snots and grots to be produced, who in turn facilitate the large fungal masses needed to grow Orks.
Natural selection proceeds in much the same way as plants. Orks that survive longer and become bigger produce more spores for longer, increasing their incidence. Weak Orks die. However since Orks also release a mass of spores when they die, living fast and dying young is a good tactic to follow as well.
Another factor is that many Orks are not unique individuals but clones from a single fungal batch. Since any given Ork can potentially have thousands of identical siblings running around being all that fussed about your unique survival isn't that important. Better to act in a way that increases the spread and dominance of Orks in general, since in that way you will help all those masses of Orks who are identical or very similar to you. This phenomenon of sacrificing your own life to aid the greater mass of those who are genetically identical/very similar to you is the basis of the social insects society. A few oddball organisms such as amoeba slime molds also act in this way, forming collective colonies of clones.