Stephen Colbert and cellist Yo-Yo Ma had their Harvard University event abruptly canceled Thursday after a fire alarm was pulled about 40 minutes into the program. Outside, hundreds of graduate student union protesters were shouting and chanting, protesting the university’s labor practices and criticizing the two for crossing their picket line. The fire department and Harvard determined there was no reason for the alarm, and the union denied having anything to do with it. The protesters say they're fighting for better pay, safe working conditions and protections for roughly 4,000 graduate workers. Colbert and Ma have not publicly commented.