Here is the letter:
http://documents.latimes.com/read-usc-faculty-letter-demanding-president-nikias-resignation/
The faculty who signed all have tenure, meaning that they can’t be fired in retaliation. Considering how much USC and other universities rely on adjuncts, this is why those tenured faculty stepped up and signed the letter. Adjuncts wouldn’t sign for fear of retaliation, nor would tenure-track faculty, nor would classified staff.