Picture a Fallout style after-the-bombs postapocalyptic world, but we humans are extinct. Nothing but ruined buildings, leftover wreckage, wandering mutants of assorted types, and abandoned hoards of supplies like armor, food, tools, and weapons...
Meanwhile, on the other side of reality, the stereotype sword-n-sorcery fantasy realm gets so sick and tired of goblins - stupid, obnoxious, violent, vulgar little wretches that breed like rats on speed and can bounce back from just about anything short of incineration or wholesale dismemberment - that they create a spell to banish the entire species to some other plane, which turns out to be this post-apoc Earth. While in their old world, they were (and still are) a pack of greedy and self-serving cowards who tended to flee in disorganized panic if even slightly outnumbered or outmuscled, Goblins, it turns out, love this new world, since it's full of free food, stuff that explodes when you poke it, and no humies or elfses or such to harass them... sure, the land is packed with ravenous monsters, but there's exploring and adventures EVERYwhere! -- Anon Guest
Some argued that it was the ultimate expression of speciesism. Others held that it was pest control. But since they argued after it was a done deal, there was no skin in the game and the entire debate was philosophical. No matter what, they all agreed that the Goblins were the Goblins problem now.
Goblins are small. They are fast. They are greedy. They breed like flies. They stink because they think hygiene is a greeting for anyone named Eugene. And when they entered the world of blasted cities, their first instinct had been to run for cover. It only took them a few hours to realise that there were no other species to come after them. That all the places still standing were full of wealth.
Shiny things. Tools. Technology. Clothing. Food. They didn't even have to hunt, the food was so plentiful. Just lying around in packets. In boxes. In cans. They had access to so much that it grew... boring.