Revert your settings and try checking things that you might done that causes that issue.
That seems to be tricky to do, anyway, I am not yet encountering that, but I hope that I can also head to that guide once I have similar issue.
That is the default configuration of bbpress, you cannot omit the /topic/ as it will always coming back and besides that points to a discussion or specific topic of the page. According to my friend from seo gold coast Australia, it is part of the systematic method for bbpress to have that one.
