The answer to this question can be found in Chapter Eight of this excellent novel. Msimangu and Kumalo have been just been persuaded to observe the boycott of transport, in spite of the distance that Kumalo must walk and his age. The man who is responsible is Dubula, of whom Kumalo says he has "a silver tongue." Msimangu then explains who Dubula is and why it is that the government fears him more than other political agitators arguing for equality:
But they say... that Tomlinson has the brains, and your brother the voice, but that this man has the heart. He is the one the Government is afraid of, because he himself is not afraid. He seeks nothing for himself. They say he has given up his own work to do this picketing of the buses, and his wife pickets the other bus rank at Alexandra.
Thus Dubula is the man the government is most afraid of because, unlike John Kumalo, he protests with no thought of benefit for himself or fear for his own personal safety. He is driven by higher and purer ideals of morality and justice, which makes him more dangerous.