the above comments don't truly do it justice.

the game uses a heat system for your base rating.

basically, you have items that can prevent raids that you can place, but they can also add to the chance of spawning a raid if there are too many too close together.

and then you have what i call the grudge rating which is basically actions you do throughout the world that anger the mobs, from cutting trees down to killing creatures in each location, all different things anger the mobs and add to the chance.

so you have your base rating, your grudge rating, and THEN it's RNG based off of those variables.

im pretty sure there's also a boss timer that plays a role as well, as well as a grace period for both the start of a world and after each raid.