**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
#
#
#
#
import datetime
import itertools
import logging
import multiprocessing
import os
import signal
import sys
import time
import warnings
from collections import defaultdict
from datetime import timedelta
from typing import Collection, DefaultDict, Dict, Iterator, List, Optional, Tuple
from sqlalchemy import and_, func, not_, or_, tuple_
from sqlalchemy.exc import OperationalError
from sqlalchemy.orm import load_only, selectinload
from sqlalchemy.orm.session import Session, make_transient
from airflow import models, settings
from airflow.configuration import conf
from airflow.dag_processing.manager import DagFileProcessorAgent
from airflow.executors.executor_loader import UNPICKLEABLE_EXECUTORS
from airflow.jobs.base_job import BaseJob
from airflow.models import DAG
from airflow.models.dag import DagModel