The part of the lord's prayer that says "our father who art in heaven" is kinda like this - father is linked to a second person conjugation. You could remove some words and make it into "father art in heaven", which you claim is ungrammatical. I'm skeptical that it was.
Conceivably it’s grammatical if Henry is vocative and the pronoun is dropped colloquially, like “Who art [thou], O Henry?” but it’s a stretch.
Today, even ignoring the dated conjugation, "who art in heaven" or "who are in heaven", does not make sense. We would switch it into the third person.
(Well, not 19th century...)
"Art thou" should be translated into modern English as "are you to be", and so works better with things (what are you going to be), or people who are alive, and have a future (who are you going to be?).
Those are probably the contexts you are thinking of.
So yes you can interpret it as "for what reason or why are you going to be" (this thing she will now explain).