My mother has late stage Alzheimer's. She rarely speaks but when she does, it sounds angry but detached from the present. What is going on?

Poor lady!

What is going on? - amateur guess, her brain is processing memories, and quite a lot of them have gone missing, unfortunately mainly the nice ones. All you can really do is provide comfort and reassurance, and hope to connect her with the immediate present where she is safe and all is well.