sometimes i'm AFK near the core and randomly a red slime is around when i come back, even if there's no slime puddle around, i noticed that sometimes they stick together, especially in the ghorm path (a group of 4-8 red slime reacting aggressively even though i didn't attack and wasn't even close to them)
I assume it's a similar raid "event" when you're in the forgotten ruins and a group of cavelings comes to your location to hunt you down