Please don't torment yourself, @ robertwills. There's no rhyme or reason to this. Their brain is broken and they can't make connections in the usual way. Maybe they see you and their brain goes to a place that was always reserved for someone they really didn't like.
A friend of mine told me her mother turned on her completely, but was fine with her sisters. It broke her heart, but she knew her mother was ill.
An old friend visited my dad who had dementia and he told my mother that the friend was an imposter. When he had a stomach ache he told my mother that two men were fighting with sticks inside his stomach. He also used to kick my husband under the kitchen table while muttering "A...hole."
Sadly, that's just how it goes. Not much we can do, but remove ourselves from unpleasant situations and not beat ourselves up.
You are special for what you are doing, your care and concern.