It looks like you’ve deleted the repo, so it’s hard to say!
GitHub determines language statistics based on the total size of the files that belong to each language.
We use the Linguist open source library to analyze source code files. Linguist supports a number of different custom overrides strategies for language definitions and vendored paths. These are commonly used for documentation, vendor files, or generated files. You can read about how to use overrides in the Linguist documentation:
It’s possible your files were accidentally counted as being vendored, perhaps?