When it happens
Why it happens
The ability to form thoughts and then make words decreases as the parts of the brain responsible for processing most forms of communication are increasingly affected. Some people utter random, inappropriate words. Some speak in a sing-song manner, though the words may be unintelligible.
What you can do
Don't bother encouraging the person to enunciate or to stop; the sounds that come out are largely involuntarily.
See if you can connect moods to the speech. For example, someone may take to muttering angry nonsense words when upset, or sing a pleasant string of nonsense words when relatively happy.