1. Balrog chases pussy dwarves out of Khazad-Dum.
2. Goblins move in because no one is occupying the space anymore.
3. 7,000 years pass.
4. Dwarves, none of whom can even remember someone who actually lived there at any point (one would be hard pressed to find an elf that could remember that either), decide they want it back.
5. Goblins are now being killed when it's really now their own land.

It takes 20 years for a squatter in the US to establish homestead rights. Goblins held Moria for 7,000. It's their land at this point. Dwarves are an invading force, goblins are the rightful defenders.