Why was the に particle used in くすぐったさに思わず本を取り落としてしまいそうになった?


> くすぐったさに思わず本を取り落としてしまいそうになった。

As far as I am concerned に cannot turn a normal noun into an adverb unless it
is an adjectival noun, so I guess this is the target particle に. In here I see
they have normalized 擽ったい with さ, but what is the target particle に doing
here?

