Why were the Philippian magistrates afraid when they learned Paul was a Roman citizen?
The magistrates feared because they had gone contrary to Roman law by punishing Paul and Silas without due process, not knowing Paul was a Roman citizen. When told, they realized they had beaten a Roman openly and now had to deal with him as a Roman.
Les teaches that Paul and Silas had been beaten and imprisoned, and after the Philippian jailer believed, the magistrates wanted to get rid of Paul and send him out of town. Paul refused, insisting that if he had been beaten openly as a Roman, they had to deal with him as a Roman. The magistrates had not known he was a Roman citizen. When told, they feared because they had gone contrary to Roman law and had punished these men without due process. Les points out that violating due process was anathema to the Romans.
And the serjeants told these words unto the magistrates: and they feared, when they heard that they were Romans.