Why the tempest can success in a single test, but fail in a batch test

Question

I used the tempest to test my openstack cloud ,I can run a single test successfully, such as

testr run -- tempest.cli.simple_read_only.test_cinder.SimpleReadOnlyCinderClientTest.test_cinder_availability_zone_list

and success in the following command

nova availability-zone-list

but when I run in batch, for example

testr run --parallel

it failed with the following info:

FAIL: tempest.cli.simple_read_only.test_cinder.SimpleReadOnlyCinderClientTest.test_cinder_availability_zone_list
tags: worker-0
----------------------------------------------------------------------
Empty attachments:
  pythonlogging:''
  stderr
  stdout
Traceback (most recent call last):
  File "/root/tempest.bak/tempest/cli/simple_read_only/test_cinder.py", line 107, in test_cinder_availability_zone_list
    self.cinder('availability-zone-list')    
  File "/root/tempest.bak/tempest/cli/__init__.py", line 84, in cinder
    'cinder', action, flags, params, admin, fail_ok)    
  File "/root/tempest.bak/tempest/cli/__init__.py", line 109, in cmd_with_auth
    return self.cmd(cmd, action, flags, params, fail_ok)    
  File "/root/tempest.bak/tempest/cli/__init__.py", line 129, in cmd
    stderr=result_err)    

CommandFailed: Command '['/usr/local/bin/cinder', '--os-username', 'xxx', '--os-tenant-name', 'xxx', '--os-password', 'xxx', '--os-auth-url', 'http://xxxx:5000/v2.0', '--endpoint-type', 'publicURL', 'availability-zone-list']' returned non-zero exit status 1

other command such as : transfer-list, all failed too.

Can anyone explain why this happened? I am sure that the tenant-name, password is correct. because the single test can pass.