What does the underscore (_) mean in Chinese?
I guess it becomes popular on Internet due to web technology favor. Most of web servers are built on Unix(or similar, like Linux) OS. Underscore is widely used as space replacement since space it's not allowed for variable name in most of the programming languages. Also, web pages usually in a hierarchical structure, and web page title usually contains the hierarchical information for SEO purpose. Thus the underscore symbol is introduced into web pages as a separator, although some programmers may choose the minus symbol instead of underscore.

As a conclusion, the underscore is nothing with Chinese, but a web stuff.