Originally posted by Samoth:

it kind of cascades, unless I am wrong about them counting towards the retribution raids. Like the skele surprise is trigger by destroying skeletons. As well being a co-participant in other raid types. Several mobs share that characteristic. So, keep in mind that trolls will come for you at a threshold, but its also possible they count for the next wave. They beget themselves, as the saying goes.

This is just wrong, raids are chosen at random, completely irrelevant to the amount that you kill (all that matters is that a troll must have died in your world at least once for the raid to unlock itself permanently)