It’s all speculation of course, because we’ve never had an economy based in UBI, which would be fundamentally transformative, but it’s based on economic theory. Essentially, some people move out of cities because they’d rather be in small towns, near families, whatever, and now they could afford it. Less population pressure in cities means less housing demand (more empty apartments for renters to choose from), and so prices decrease in response. If you’ve ever tried subletting a room, it’s apparent that the price you can get is heavily reliant on how many people want that room.