Why does yum update fails with error "sqlite3.OperationalError: database is locked" ?

Issue

• The command yum update fails with error "sqlite3.OperationalError: database is locked"

# yum update

Loaded plugins: product-id, rhnplugin, search-disabled-repos, security, subscription-manager
This system is receiving updates from RHN Classic or RHN Satellite.
Repository 'simp-project_6_x_dependencies' is missing name in configuration, using id
Setting up Update Process
Resolving Dependencies
--> Running transaction check
---> Package puppet-agent.x86_64 0:1.10.10-1.el6 will be updated
---> Package puppet-agent.x86_64 0:5.3.6-1.el6 will be an update
--> Finished Dependency Resolution

Dependencies Resolved

============================================================================================================================================
 Package                           Arch                        Version                            Repository                           Size
============================================================================================================================================
Updating:
 puppet-agent                      x86_64                      5.3.6-1.el6                        puppet-col-el6                       20 M

Transaction Summary
============================================================================================================================================
Upgrade       1 Package(s)

Total download size: 20 M
Is this ok [y/N]: y
Downloading Packages:
puppet-agent-5.3.6-1.el6.x86_64.rpm                                                                                  |  20 MB     00:00
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
Traceback (most recent call last):
  File "/usr/bin/yum", line 29, in <module>
    yummain.user_main(sys.argv[1:], exit_code=True)
  File "/usr/share/yum-cli/yummain.py", line 298, in user_main
    errcode = main(args)
  File "/usr/share/yum-cli/yummain.py", line 227, in main
    return_code = base.doTransaction()
  File "/usr/share/yum-cli/cli.py", line 588, in doTransaction
    resultobject = self.runTransaction(cb=cb)
  File "/usr/lib/python2.6/site-packages/yum/__init__.py", line 1537, in runTransaction
    self.skipped_packages, rpmdb_problems, cmdline)
  File "/usr/lib/python2.6/site-packages/yum/history.py", line 955, in beg
    self._trans_cmdline(cmdline)
  File "/usr/lib/python2.6/site-packages/yum/history.py", line 917, in _trans_cmdline
    if cur is None or not self._update_db_file_2():
  File "/usr/lib/python2.6/site-packages/yum/history.py", line 1590, in _update_db_file_2
    executeSQL(cur, "PRAGMA table_info(trans_skip_pkgs)")
  File "/usr/lib/python2.6/site-packages/yum/sqlutils.py", line 166, in executeSQLQmark
    return cursor.execute(query)
sqlite3.OperationalError: database is locked