**Apache Airflow version**: 2.0.1


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): 1.18.14

Environment:

Cloud provider or hardware configuration: Azure
OS (e.g. from /etc/os-release):
Kernel (e.g. uname -a):
Install tools:
Others:

**What happened**:

An occasional airflow tasks fails with the following error

```
[2021-06-21 05:39:48,424] {local_task_job.py:184} WARNING - State of this instance has been externally set to up_for_retry. Terminating instance.
[2021-06-21 05:39:48,425] {process_utils.py:100} INFO - Sending Signals.SIGTERM to GPID 259
[2021-06-21 05:39:48,426] {taskinstance.py:1238} ERROR - Received SIGTERM. Terminating subprocesses.
[2021-06-21 05:39:48,426] {bash.py:185} INFO - Sending SIGTERM signal to bash process group
[2021-06-21 05:39:49,133] {process_utils.py:66} INFO - Process psutil.Process(pid=329, status='terminated', started='04:32:14') (329) terminated with exit code None
[2021-06-21 05:39:50,278] {taskinstance.py:1454} ERROR - Task received SIGTERM signal
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/airflow/models/taskinstance.py", line 1112, in _run_raw_task
    self._prepare_and_execute_task_with_callbacks(context, task)
  File "/usr/local/lib/python3.7/site-packages/airflow/models/taskinstance.py", line 1284, in _prepare_and_execute_task_with_callbacks
    result = self._execute_task(context, task_copy)
  File "/usr/local/lib/python3.7/site-packages/airflow/models/taskinstance.py", line 1309, in _execute_task
    result = task_copy.execute(context=context)
  File "/usr/local/lib/python3.7/site-packages/airflow/operators/bash.py", line 171, in execute
    for raw_line in iter(self.sub_process.stdout.readline, b''):
  File "/usr/local/lib/python3.7/site-packages/airflow/models/taskinstance.py", line 1240, in signal_handler
    raise AirflowException("Task received SIGTERM signal")
airflow.exceptions.AirflowException: Task received SIGTERM signal
```

There is no indication as to what caused this error. The worker instance is healthy and task did not hit the task timeout.

**What you expected to happen**:

Task to complete successfully. If a task fad to fail for unavoidable reason (like timeout), it would be helpful to provide the reason for the failure.

**How to reproduce it**:

I'm not able to reproduce it consistently. It happens every now and then with the same error as provided above.

I'm also wish to know how to debug these failures 



airflow/jobs/scheduler_job.py
dag_run.state = State.RUNNING
            dag_run.start_date = timezone.utcnow()
            if dag.timetable.periodic:
                
                
                
                
                
                expected_start_date = dag.get_run_data_interval(dag_run).end
                schedule_delay = dag_run.start_date - expected_start_date
                Stats.timing(f'dagrun.schedule_delay.{dag.dag_id}', schedule_delay)
        for dag_run in dag_runs:
            dag = dag_run.dag = self.dagbag.get_dag(dag_run.dag_id, session=session)
            if not dag:
                self.log.error("DAG '%s' not found in serialized_dag table", dag_run.dag_id)
                continue
            active_runs = active_runs_of_dags[dag_run.dag_id]
            if dag.max_active_runs and active_runs >= dag.max_active_runs:
                self.log.debug(
                    "DAG %s already has %d active runs, not moving any more runs to RUNNING state %s",
                    dag.dag_id,
                    active_runs,
                    dag_run.execution_date,
                )
            else:
                active_runs_of_dags[dag_run.dag_id] += 1
                _update_state(dag, dag_run)
    def _schedule_dag_run(
        self,
        dag_run: DagRun,