Take on this - Trolls, like many insect species, are parasitoids.
We see lusii raising trolls, but we don't see how they breed themselves, and we don't see Lusii parents in pairs as well. The reason for this - Troll grubs are not raised by a single Lusus, but by two.
As a grub in the brooding caverns, the troll parasitizes the body of the opposite gendered Lusus - and develops amd grows inside their body. By the time the grub has developed rudimentary humainoid shape, it emerges from the body of the lusus (using its horns to cut through the body), killing that host. However, because the grub has been feeding off the Lusus, it smells the same, and as we know smell is an important component of Troll-Lusii bonding. The partner Lusus, connecting the troll with its now deceased mate, then continues to display parental care on the troll as a young imago.