In version 2.8.0 we had tried to fix an issue where people with a custom vendor dir could not use the phar distribution by allowing phpDocumentor to scan the composer.json in the current working directory if no other sources could be found.
This introduced a fatal issue where if you tried to use the phar distribution of phpDocumentor from a folder with a composer.json that it used that composer.json to autoload its library files. This of course caused a fatal error.
This hotfix release is all about reverting the offending change; after this the fatal errors caused by this are history.
Our apologies for the inconvenience and have fun documenting!