When Vespasian led the Roman legions under Nero into Galilee to wipe out the Zealot threat, he systematically went to each town and garrison and secured or destroyed them. As each town was defeated, a group of escaping Zealots would make their way south, generally to Jerusalem or one of the fortified areas south of there, like Masada.
Again, the word ‘preaching’ here meant the pronouncement of an official Imperial decree, not some effort at rhetoric or propaganda (beyond what was already in the decree, of course). So “casting out demons” may simply represent the zealots escaping to Judea.