Say an explorer lands on a previously undiscovered land, filled with humans, humanoids, wildlife, elementals, undead, demons... you name it.
One would expect that these existing inhabitants would have their own power struggle. For example, the humans would be fending off the undead while also fighting against other humans over resources and hunting wildlife for food; the demons would be fighting against the elementals in a struggle to control points imbued with powerful magicks; the wildlife would have their own fights over food and territory, while the most powerful of them raid human encampments for their food, and so on.
And yet, as soon as the explorer shows up, all these beings seem to immediately forget their own struggles; instead, they work together in an effort to kill the explorer. Indeed, opposing tribes of humans fight together as one, leading the ferocious beasts they once feared and even the undead into battle, while diametrically opposed elementals and demons aid each other in battle, such as a flame demon imbueing a stone elemental with the power of fire.
Aside from the cop-out explanations such as "they are all under the influence of a higher being", how can this behaviour be explained?