For the first time in seven years, New York City officials expect the number of foreign visitors to decrease, a drop they attribute to the protectionist policies and rhetoric of President Trump—saying that those moves are scaring off many of visitors who have helped fuel the city’s robust growth since the last recession.

我觉得可以理解为a drop前省略了which is。这样，逗号之后就是一个非限制性定语从句，which的先行词是to decrease。which引导非限制性定语从句时，先行项可以是整个主句内容，也可以是主句的一部分。例如：

Tom dances well, which I don't. which的先行项为谓语dances well。

Mary is clever, which her sister is not.  which的先行项为形容词clever.

Tom promised to help me, which he did. which的先行项为不定式to help me.

这类非限制性定语从句其实已经不是定语了（虽然传统语法仍叫它们定语），而是对主句或主句的一部分内容作补充说明。起的是补充性状语的作用。

在逻辑上，a drop对应的是decrease，但我们不能说一个名词是动词 的同位语。只能是补充说明。语法上把不好归类的句子成分就叫做状语。我们不妨称a drop...整个结构是一个名词短语作补充性状语。