Why is font-lock-keyword-face highlighting text within double quotes in Emacs?

I am trying to set up a major mode in Emacs where I would like to highlight certain keywords. Using a template from this page: http://ergoemacs.org/emacs/elisp_syntax_coloring.html I tried:

(setq testing-font-lock-keywords
  `((font-lock-keyword-face)
))

(define-derived-mode testing-mode fundamental-mode
  "testing file mode"
  "Major mode for editing test files"

  (setq font-lock-defaults '(testing-font-lock-keywords))
  (setq mode-name "testing")
)

(provide 'testing-mode)

If I use this mode on a simple test file, and type "hello" the text hello is marked in different color. That is, any text within double quotes is highlighted. Why is this happening?

I think it is related to the variable font-lock-keyword-face. But if I type C-h v and font-lock-keyword-face it says:

font-lock-keyword-face is a variable defined in `font-lock.el'.
Its value is font-lock-keyword-face

Update

It seems like it is not related to font-lock-keyword-face anyway, since defining testing-font-lock-keywords like:

(setq test-keywords '("TEST"))
(setq testing-font-lock-keywords
  `((,test-keywords)))

gives the same behavior.

Solution

This is directed by variable `font-lock-syntactic-face-function'