It created a new folder with the named after the file name and moved that track to that folder.
The backslash is a special character for filenames to create folders. Use the $validate function and $replace() if you do not want to use this feature.
Depending on the Operating System, there are a few characters not allowed to be member of the filename respectively of the filepath.
[F] $validate() broken?
For example the Slash (ASCII 47) and the Backslash (ASCII 92) are forbidden characters.
You are responsible for your own to make sure that your filename follows the rule.
The Mp3tag function $validate can help to make this effort simple.
See Mp3tag help manual, example and notes at end of section "Tag - Filename".
The Backslash has a special meaning when formatting the pseudo tag-fields %_filename% or %_directory% and allows to set also a new folder name.
That is one of the special big features of Mp3tag.