I could be wrong but, your gonna get raids after completing a time period in that Biome. Depending on how much time you spend in one biome depends on what raid you will get and what biome is close to you as well.

Ive started new seed map and have not done any bosses or hung any boss heads and ive still got raids. So Im confused unless this action depends on the players level and past actions from another map seed. Who knows?