Diplomatic action
Over a period of some years, the Therrans stop interacting politically with the Garruw.
At some point after the rebuilding from the Upheaval, the Garruw start to pull back from the Therrans. The Dome is a more rigid space than before, more of a specific bubble, and it's difficult to interact with that space with the breathing issues. It's not quite xenophobia on either side, but in the aftermath of the earthquake it's clear that both species have chosen to focus on themselves, and there's at least some resentment over the other side not helping them.