underscores and lowercase was the style perferred by Bjarne Stroustrup in “The C++ Programming Language”. If I recall correctly he had made a statement along the lines that underscores in names were to be preferred because it was more readable to an international community where english is not the primary language. I have no idea if his opinion is true or not, but I’m guessing that’s the origin.
Here’s a link to his FAQ where he discusses this very topic:
http://www.stroustrup.com/bs_faq2.html#Hungarian
Snippet explaining what you were interested in in particular:
I prefer to use underscores to separate words in an identifier (e.g, element_count) rather than alternatives, such as elementCount and ElementCount. Never use names with all capital letter (e.g., BEGIN_TRANSACTION) because that’s conventionally reserved for macros. Even if you don’t use macros, someone might have littered your header files with them. Use an initial capital letter for types (e.g., Square and Graph). The C++ language and standard library don’t use capital letters, so it’s int rather than Int and string rather than String. That way, you can recognize the standard types.