I've read some spiritual warefare theories on this subject. I would first say that Jesus told the demons to go into the pigs because the demons asked nicely. The demons then drove the pigs off the cliff to get as far away from Jesus as fast as they could.
This passage could suggest that demons need something to cling onto, or else they are "open game" for angelic attack. This is all pure speculation of course. But if you look at other scriptures, such as when Daniel's angel was confronted by the prince of persia, you find that there indeed to appear to be "rules" or "laws" that these spiritual beings follow to interact with each other.
The bible also talks about principalities, both angelic and demonic. Daniel's angel obviously had to fight his way out, whereas the demons were either not in their territory, or the demonc prince of the region and his staff ran like girls when Jesus paid a visit.
Then I heard the voice of the Lord saying, "Whom shall I send? And who will go for us?" And I said, "Here am I. Send me!" Isaiah 6:8
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