Cassius is a Shakespearean name with the feel of antiquity that is coming into fashion. Cassius Clay was an abolitionist and the birth name of Muhammad Ali. It was chosen for their sons by singer Bobby Brown and Getty heir/actor Balthazar Getty. Vanessa Marcil and Brian Austin Green used the nouveau Kassius spelling for theirs. With these namesakes, it's no surprise this name has become popular in recent years.
Cassius is one of a number of Cas-starting names drawing attention, including Caspian, Cassian, Castor, Casper, Cashel, and Cash. Cass and Cash are both accessible nicknames.
Americans tend to pronounce the name CASH-us while Europeans, including the British, say CASS-ee-us.