The why behind the sale decline

"What worries Gruhn is not simply that profits are down. That happens in business. He’s concerned by the “why” behind the sales decline. When he opened his store 46 years ago, everyone wanted to be a guitar god, inspired by the men who roamed the concert stage, including Clapton, Jeff Beck, Jimi Hendrix, Carlos Santana and Jimmy Page.Now those boomers are retiring, downsizing and adjusting to fixed incomes. They’re looking to shed, not add to, their collections, and the younger generation isn’t stepping in to replace them.

Gruhn knows why." (The Washington Post.)

Is why a noun in the above?

Is the object of the preposition the “why” behind the sales decline a small clause in the sentence He’s concerned by the “why” behind the sales decline? (I think it is. I interpret it as a verbless clause with implied "which/that is", i.e., He’s concerned by the “why” [that is] behind the sales decline.)