First of all, welcome to the website.
Next, strange question!
I don't know about this one. Scripture is silent on it, so I don't like to put into scripture what isn't there, or take away from it for good reason, Rev 22v18-19.
What I do know, from scripture, is that the Jews were not allowed to eat certain meats, and pork was one of them, so what were they doing with pigs anyway?
As for why the pigs jumped off the cliff-look at what the demons had been doing to the man they just left.
We know it says in Romans 8 that the whole creation groans, and pigs are part of God's creation, so to be engulfed suddenly by demons must have driven the pigs crazy.
That is just an assumption of mine.
What happened to the demons after? Who knows?
Scripture does not say, so we cannot, we can only assume.
Maybe they did go roaming around looking for new territory to inhabit.
Matt 12v43, 'When an unclean spirit goes out of a man, he goes through dry places, seeking rest, and finds none.
vs 44, 'Then he says, "I will return to my house from which I came". and when he comes, he finds it empty, swept, and put in order.
vs45, 'Then he goes and takes with him seven other spirits more wicked than himself, and they enter and dwell there, and the last state of that man is worse than the first. So shall it also be with this wicked generation.
Whilst Jesus is speaking here about that generation, He also mentions about demons leaving someone.
If the person has not filled the place with the redemption that is found in Jesus Christ, then that person is desolate and vunerable, and the demon, plus 7 others, can enter the individual.
That's all I have, but I know I have missed things that others will point out.
God bless.