Expositor's Bible Commentary Commentary Mark 5:13

"And he gave them leave. And the unclean spirits came out, and entered into the swine: and the herd rushed down the steep into the sea, [in number] about two thousand; and they were drowned in the sea." — Mark 5:13 (ASV)

What caused the stampede of the pigs was the entrance of the demons into them. They were bent on destruction. Not having been able to destroy the man, they destroyed the pigs. Demons are emissaries of Satan, the Destroyer. Why the sovereign Lord allowed this destruction is uncertain, but perhaps he wanted to give tangible evidence to the man and to the people that the demons had actually left him.