分析一下语法和句子结构。

angered at the prospect of seeing their pension plans replaced with less generous versions做原因状语.angered 被激怒

这个句子有两个动词，但没有用and连接，说明一个动词是谓语，另一个动词就是非谓语，在这里就是状语。

句子的主要部分是 Pilots at the airline vowed to use legal means to fight such a move.

angered at the prospect of seeing their pension plans replaced with less generous versions在句子中作状语，

Pilots 和angered的关系是被动的，被激怒，所以是用过去分词作状语，这是非谓语动词的用法

replaced with less generous versions 过去分词短语作后置定语，修饰前面的名词their pension plans， 可以改成定语从句：their pension plans which/that were replaced with less generous versions

这绝对不是一个非限制性定语从句。Pilots是主语，如果是定语从句，关系代词在从句中作主语成分的时候是不能省略的

还有在原因状语从句中，前面的引导词是可以省略的如：
Father's being ill, she had to stay at home to look after him.（因为）爸爸生病了，她不得不呆在家照顾他。爸爸生病就是原因状语。

Pilots（主语）at the airline, angered at the prospect of seeing their pension plans replaced with less generous versions（定语从句省略whoare）, vowed to use legal means to fight such a move（谓语部分）.