Jesus had just seen a man who had sold himself
to the devil. He sensed Satan was around to
possess and destroy Peter too.

At times Satan did put words into Peter's mouth
so that for an instant he became his spokesman,
"Then Peter took him, and began to rebuke him,
saying, Be it far from thee, Lord: this shall not
be unto thee.
But he turned, and said unto Peter, Get thee
behind me, Satan: thou art an offence unto me:
for thou savourest not the things that be of God,
but those that be of men." Matthew 16:22-23