(12)  And all the devils besought him, saying, Send us into the swine, that we may enter into them. (13)  And forthwith Jesus gave them leave. And the unclean spirits went out, and entered into the swine: and the herd ran violently down a steep place into the sea, (they were about two thousand;) and were choked in the sea. 

Answer: I Believe that there are THREE POSSIBLE reasons for this, Let me explain…

#1… The demons THOUGHT that they could TRICK Jesus by having him cast them into the pigs, so when the pigs drowned and died they (demons) thought they would be free to go about in possessing other people which was their whole game plan, but it BACK FIRED.

NOTE: Although NOT MENTIONED it is possible that Jesus CHAINED these wicked demons (fallen angels) to this sea to PREVENT them so they COULD NOT go about possessing other people and reserved them in prison or confinement until their day of Judgement.

(Jude 1:6) And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.

(2 Peter 2:4) For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment.

NOTE: the word hell in 2 Peter 2:4 is the Greek word TARTAROO and means ETERNAL TORMENT this is the ONLY PLACE in scripture that the word hell is translated as TARTAROO and it is a special place of torment RESERVED for the MOST WICKED of demons(fallen angels).

#2… This is a VERY POSSIBLE scenario as well and more likely the REAL REASON
.
Under the Old Testament Law, Jews WERE NOT ALLOWED to eat pork or even touch it. There were possibly Jews that were marketing pigs for sale even though THEY KNEW it was unlawful for them to have pigs. So when Jesus came along and saw this He caste the demons into the pigs and stopped their money making racket.

Leviticus 11:7-8 And the swine, though he divide the hoof, and be clovenfooted, yet he cheweth not the cud; he is unclean to you. Of their flesh shall ye not eat, and their carcase shall ye not touch; they are unclean to you.
Deuteronomy 14:8 And the swine, because it divideth the hoof, yet cheweth not the cud, it is unclean unto you: ye shall not eat of their flesh, nor touch their dead carccase.

Deuteronomy 14:8 And the swine, because it divideth the hoof, yet cheweth not the cud, it is unclean unto you: ye shall not eat of their flesh, nor touch their dead carcase.

#3… Or it could be just simply the fact that these Jews had DISOBEYED the Law, therefore incurring Jesus judgement.

But whatever the situation was we are just not told.

In Conclusion

It BEHOOVES us to obey the Lord and his Word if you want his blessings rather than his correction.

Hebrews 12:5,6 And ye have forgotten the exhortation which speaketh unto you as unto children, My son, despise not thou the chastening of the Lord, nor faint when thou art rebuked of him: 6 For whom the Lord loveth he chasteneth, and scourgeth every son whom he receiveth.

The MOST IMPORTANT thing to know IS Jesus Christ as savior, just simply believe that Jesus Christ went to the cross and DIED and shed his blood, was BURIED and three days later RESURRECTED from the dead to pay for your sins.

-Christ DIED for you – Romans 5:8

-Christ PAID your sins in full – 2 Corinthians 5:21,1 Peter 2:24

– Christ AROSE as proof that God accepted Christ payment for your sins – 1 John 2:2, Matthew 28:6, 1 Peter 3:18

-BELIEVE (Trust In Completely) in Christ ALONE as the complete payment of sin for salvation – John 3:16, Acts 16:31, Ephesians 2:8,9

“For the wages(payment) of sin is death (eternal torment in hell); but the GIFT(salvation) of God is eternal life THROUGH Jesus Christ our Lord”. Romans 6:23

IT IS THAT SIMPLE – THE GOSPEL TRUTH