[Olaf Lederer]

The first “forum” is the forum’s folder and the second is used to identity a (sub)forum.

The same way like a tag or topic is identified. You can change the forum’s folder name, that’s all.

the structure make sense:

https://bbpress.org/forums/ (forums home)

https://bbpress.org/forums/forum/troubleshooting (the troubleshooting forum)