Raids are directly tied to the bosses, you get the Raid for your "next" boss only after defeating the current boss.

example, you can go basically forever on a map without ever experiencing any Raid other than Eikthyr Rallies Creatures. just don't defeat any bosses and you'll be good, only drawback to this is you won't ever progress into better tech (Pickaxes,improved armors, etc) by doing this.