Trolls have a complicated reproductive cycle. It's probably best not to examine it in much detail.
The need to seek out concupiscent partners comes with more urgency than typical reproductive instincts. When the IMPERIAL DRONE comes knocking, you had better be able to supply genetic material to each of his FILIAL PAILS. If you have nothing to offer, he will kill you without hesitation.
The genetic material - WITHOUT GOING INTO MUCH DETAIL - is a combinative genetic mix from the matesprit and kismesis pairs, respectively. The pails are all offered to the mother grub, who can only receive such precombined material. She then combines all of it into one incestuous slurry, and begins her brooding.
This doesn't mean the initial combination was for naught, however. In the slurry, more dominant genes rise to the fore, while the more recessive find less representation in the brood. Especially strong matesprit and kismesis pairings yield more dominant genetic material. The more powerful the complement or potent the rivalry, the more dominant the genes.
TROLL REPRODUCTION SURE IS WEIRD. We all take a moment to lament how pedestrian the human reproductive system is, and further lament that the phrase "incestuous slurry" is not a feature of common parlance in human civilization.