In C++,

1. identifiers starting with an underscore, followed by a capital character
2. identifiers having two consecutive underscores anywhere
3. identifiers in the global namespace starting with an underscore

are reserved to the implementation. (More about this can be found here.) Rather than trying to remember these rules, many simply do not use identifiers starting with an underscore. That's why the trailing underscore was invented.
However, C++ itself is old, and builds on 40 years of C, both of which never had a single company behind them, and a standard library that has "grown" over several decades, rather than brought into being in a single act of creation. This makes for the existence of a lot of differing naming conventions. Trailing underscore for privates (or only for private data) is but one, many use other ones (not few among them arguing that, if you need underscores to tell private members from local variables, your code isn't clear enough).

As for getters/setters - they are an abomination, and a sure sign of "quasi classes", which I hate.