2.Why did the Apostle Paul leave Antioch and go up to Jerusalem?

Because of a Divine revelation.

Perhaps the Lord Jesus had showed Paul by revelation that the issue of the Law, particularly circumcision (Acts 15:5), would arise to trouble the central church in Jerusalem. The issue of the Law of Moses, and its relationship to the Gospel, was raised by certain sincere but misguided Pharisees who had received Jesus as Christ.

The Lord directed Paul to attend this conference because the testimony of him and Barnabas was important to the decision that was to be made by the leaders of the church—all of whom were Jews (Acts 15:12).