First I was thinking this way:
https://www.usingenglish.com/forum/ask-teacher/109526-not-you-but-i.html#post537112
At a later time, however, while I was reading TG, I stopped short in amazement:
[Who the reason are] is you. :cross:
[Who the reason is] is you. :tick:
[subject] = [Who the reason is]; [The reason (= singular 3rd person) [STRIKE]are[/STRIKE]
is who]
who = the reason; the reason/who is you
It is you [who are the reason].
It is you who is the reason.
One of this pair of sentences is the extraposed variant of "Who the reason is is you."
Before the original subject's (the reason is Who) moving out of S into VP happened, 'the subject-clause' had a verb in singular.
Question: What changed the verb from 'is' into 'are'? And why?
If in
It is you [who *be the reason].
the bracketed part is a relative clause, then it follows that the relative clause together with what it describes, 'you', is a single nominal phrase contituent and can undergo NP movement.
You who *be the reason is. :cross:
Who *be the reason is you. :tick:
They, 'you' and 'who *be the reason', do not go hand-in-hand in the extraposed and 'non-extraposed' sentences, as the NP movement illustrates. If this is so, and this is so, then 'who' is not the antecedent of 'you'. No antecedent means no agreement. The noun phrase 'who' has to agree is:
Homer and Harman probably are two widely-acclaimed grammarians, but without knowing their motivation for their choice of 'are' and the lack of a principled account in defence of it, I can hardly yield to their view