"Because you have said, 'These two nations and these two lands will be mine, and we will possess them,' although the LORD was there,Apparently the people of Seir, wherever that is, had ambitions on Israel and Judah. They figured the land was ripe for the taking, after Assyria and Babylon finished ransacking it. While I don't know where Seir is off the top of my head, I assume it was close by, and therefore a good choice for them to expand into.
However, the land was not empty. God still controlled it, and He wasn't going to just let someone waltz in and take what He'd given to others. It's not clear whether the people of Seir knew God, or thought He'd abandoned the place. But it sounds like they're going to learn soon, and the hard way.