Economy of scale. Given fixed size of the UBI check, you want to live where services are cheapest. And services will be cheapest in places where more people are serviced together, because that cuts down on distribution costs and other such overheads.
Also, areas where people who have more money than just UBI live, would have higher prices simply by virtue of there being more money in the local economy. This isn't really dissimilar from the existing arrangement, where rich areas are expensive to live in, and so there's natural geographic segregation. But UBI would boost this into high gear.
To clarify, I am a strong proponent of UBI personally. But I think we should be really careful with these kinds of unintended side effects, and try to anticipate and manage them before they cause a bigger mess than what we're trying to address.
Also, have a look at this: https://medium.com/migration-issues/should-a-universal-basic...