It's very complex to explain what is behind her reasoning for studying hard until now.
ご質問ありがとうございます。
「what is behind」はこのシチュエーションで使いたいなら、下記の英文の通りになります。
「It's very complex to explain what is behind her reasoning for studying hard until now.」
「what is behind her reasoning」は「理由の背景にあるもの」を表します。
別の例文:
He's acting very nice recently. I wonder what's behind that generosity.
彼は最近優しすぎる。そんな親切さの理由はなんだろう!
ご参考になれば幸いです。