"The forced-install behavior has been witnessed at least three times by Windows Secrets editors, but Microsoft says its procedure for Automatic Updates hasn't changed in the last 10 months.
The behavior seems to occur only if a Windows user has Automatic Updates configured to "download updates but don't install them" or "notify me but don't install them." If updates are scheduled to occur automatically, with no notice to users, the silent installation of updates would be expected."
http://WindowsSecrets.com/comp/090625