The believers sent Saul to Tarsus because some Jews tried to kill him.

Acts 9:29-30

29 Saul was also speaking about Jesus with Jews who spoke Greek, and he was debating with them. But they were continually trying to think ◄of a way to kill him/of how they could kill him►. 30 When the other believers heard that those Jews were planning to kill him, some of the believers took Saul down to Caesarea city. There they arranged for him to go by ship to Tarsus, his hometown.