I've thought long and hard on this, and I can't yet find how automation causes any need for UBI.

The idea that automation reduces employment opportunities is a legend. It's never happened, and nobody can tell if it will happen in the future.

Theoretically, in my estimation, if it did happen -- where automation made people unable to produce for greater livelihoods -- it would necessarily mean that the people already get better stuff from the automation than that which they can work for. This is probably inevitable to a degree in the far, far distant future with unbelievable technology and people have little need to work.