文法・語法チェック---8

Harry watched them go, feeling slightly uneasy. It had just occurred to him that Mr and Mrs Weasley would want to know how Fred and George were financing their joke shop business when, as was inevitable, they finally found out about it.(Phoenix)

関係代名詞as
as was inevitable
'they finally found out about it'
asの指す内容はthey以下の文章全体

同様の例文を辞書から引くと
As might be expected, a knowledge of psychology is essential in the advertising business. 当然のことであるが, 広告業には心理学の知識が絶対に必要である.

以下の文のasは、前文を受ける。
He was late, as is often the case with him. 遅刻した, 彼にはよくあることだが.

その他この箇所では、
１．watch＋O+原形不定詞

２．feeling分詞構文「同時動作」

３．occur to:It ～s…to do，It ～s…that… の形で] 〔人(の心)に〕浮かぶ，思い出される.