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> 貴様、何をにやついておるのだ…？  
>  What are you grinning at, you bastard?

This sentence (asked in [this
question](https://japanese.stackexchange.com/q/44165/5010)) seemed fairly
natural and simple to me...until I realized **にやつく is an intransitive verb**.
[×]彼は私をにやついた is ungrammatical. Instead, you have to say something like
彼は私を見てにやついた.

Then why is the first sentence ever possible? Well, after some pondering I
came up with a similar expression:

> 何をぐっすり寝ているの、早く起きなさい！

So apparently these 何を with intransitive verbs mean _why_ rather than _what_.
But is that really true? I found a page where [one person says 何を can mark a
reason/trigger](http://www.guidetojapanese.org/forum/viewtopic.php?pid=33754#p33754).
But I'm not totally convinced, since all I could find was this short comment.
I'd appreciate if someone explain this or point me to a longer
article/reference. Should this be considered as a fixed expression like
English "how come"?

(Just to be sure, as a native speaker of Japanese, I know these examples using
何を are natural, but I'm just having trouble getting a nice grammatical
explanation.)
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question
particle-を interrogatives
Using 何を with intransitive verbs (eg 何をにやついているの?)
あまり詳しい説明ではないのですが、明鏡国語辞典によりますと・・・

> なに【何】  
>  〘代〙  
>
> ❸《「ーを・・・か？」「ーを・・・のだ！」など、疑問・反語・詰問などを表す自動詞文で》不審の気持ちで、事態成立の基盤を問う。また、その不当性を非難する。どんな理由で。なぜに。なんで。
> 「何を泣いているのか？」「何をためらうことがあろうか」「何をぐずぐずしてるんだ！」  
>  （語法）他動詞の場合は、～ヲに対する普通の疑問を表す。「何を読んでるの？」
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2017-03-08T03:28:37.210
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answer
(Adding my own answer for those who may run into this problem in the future)

@Shoko's answer confirmed my hypothesis. You can use 何を with intransitive
verbs, and it actually means "why", "how dare you", "what makes you", etc. It
usually has some dubious/accusatory tone. You can consider it a special
construction.

The following example using an intransitive verb 驚く shows the difference
between 何を and 何に.

  * **何に** 驚いているんですか？  
What are you surprised at?  
(I know you are surprised at something, but I don't know what you are
surprised at.)

  * **何を** 驚いているんですか？  
Why are you surprised (at this)?  
(I know what you are surprised at, but I don't understand why you are
surprised. "It's no wonder", "Isn't it obvious?")

Unsurprisingly, を can be omitted in casual speech: 何驚いてるんですか？
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answer