に indicating the source of causation?

Question

Consider this sentence

speaker is talking to himself about how to deal with an adversary (X), he has so far tried to hire a 暴漢 to beat up X, and a 探偵 to snoop on X, both of which have failed.

暴漢に襲わせても返り討ち、探偵を雇っても不発……。

in this case would interpret this as:

• (俺が)暴漢に(Xを)襲わせても返り討ち、探偵を雇っても不発……。

Compared to

傾いた太陽の橙色が辺り一帯を神々しく染め上げて、風で舞った花びらたちがまるで雪のように煌めいている。

その中心で美しく長い髪を風になびかせながら、優しさに満ち溢れた瞳を細めて俺を見つめる女の子。

and

だが、どんなに身をよじって暴れても、男の人の力には敵わなかった。

興奮に目を血走らせたおじさんが、まるで嘲笑うようにべたついた手で....

At first glance it would appear

風 "causing" 髪をなびかせる - i don't think 髪を風に(して)なびかせる applies here

興奮 "causing" 目を血走らせた

As far as i can tell, that does not seem grammatical. But i can't easily add unspoken (...) to these for them to make sense.

compare to:

冷たい風が吹き抜け、短い髪をなびかせる。

where 風 is no doubt 短い髪をなびかせる'ing.

How is に and the following causative expressions used here?

thanks

Answer 1

Both なびく and 血走る are "intransitive verbs that do not take を" when used plainly. When you form the causative form from these verbs, you have to mark the agent (causee) with を, not に (See this rule). Here に plays a role that has nothing to do with the grammar of causation (I believe you already know this is 原因の「に」).

    髪がなびく。            目が血走る。     (simple intransitive usage)
風に髪がなびく。      興奮に目が血走る。     (simple intransitive usage with に)
    髪をなびかせる。        目を血走らせる。 (causative usage)
風に髪をなびかせる。  興奮に目を血走らせる。 (causative usage with に)

Don't confuse this with causative forms made from transitive verbs, where the agent is marked with に (e.g. 息子にお菓子を食べさせる, 暴漢に奴を襲わせる).