Scheduler Job killed for unknown reason

Sometimes when DBMS_SCHEDULER job is STOPPED and the USER_SCHEDULER_JOB_LOG.ADDITIONAL_INFO says only that "Job slave process was terminated" you may find the real reason in ERR$ table

ORA-00028: your session has been killed
ORA-04036: PGA memory used by the instance exceeds PGA_AGGREGATE_LIMIT

if your session has been killed during execution of sme DML statement.