I am struggling to understand the syntax of this sentence that I found in a grammar book.
あまりの言葉にあきれてものが言えませんでした。
I could not say anything because of the outrageously unkind words.
I read it as "not many words に outrageous thing が was unable to speak."
In other words, the pattern used to say "because of B I was C(action) A" would be "AにBがC(action)." Is this the correct understanding of this grammar?
The に doesn't really mean 'because' there. It's just the particle the verb あきれる takes. You're making the mistake of trying to parse beyond sentence boundaries.
The basic structure of the sentence is that there are two clauses, which are joined by the て form.
Sentence 1:
あまりの言葉にあきれて
Shocked by (someone's) overly harsh words
Sentence 2:
ものが言えませんでした。
I was at a loss for words
The て form when joining two sentences sometimes implies reason: 「明るくて、眠れない」 it's so bright that I can't sleep
And it is the て form joining the two sentences that carries the connotation of 'because'.
I was so shocked by [his] excessive harshness that I couldn't speak
By the way, ××にあきれて、ものが言えない returns quite a few results on a web search and seems to be somewhat of a fixed phrase. Here's a sample from 研究社 新和英中辞典
彼のばかにはあきれて物が言えない。
His stupidity really staggers me.
Some pages (like this and this) don't seem to explain this usage, but に can actually mark a reason.
デジタル大辞泉 lists this as the seventh meaning of に:
７ 動作・作用の原因・理由・きっかけとなるものを示す。…のために。…によって。「あまりのうれしさ―泣き出す」「退職金をもとで―商売を始める」
This page explains this usage as the 13th (!) meaning of に:
１３ 原因・理由を表わすことがあります。
(61)人の多さにびっくりしました。
I may be wrong, but I feel this type of に is used with certain verbs related to human feelings. There seems to be a scientific article about this (unfortunately I couldn't access the content).
To break down あまりの言葉にあきれてものが言えませんでした:
