Jesus drew a crowd in this Gentile region. They were impressed by Jesus and afraid. Luke does not tell us why fear swept over them. They could have been afraid of Jesus' power. They could have been afraid of being judged. Or they could have been afraid because they lost all of those pigs and were hit in the pocketbook.
Fear can keep us from trusting God. Fear can keep us from giving financially to help others or to support the ministry of the church. But God's perfect love overcomes and drives out fear. We are free to live in faith, not fear.