Comment 4 for bug 1602536

I believe this happens because unattended-upgrades gets and releases the lock a couple times (such as between installing upgrades and then removing now no longer required automatically installed packages) and dies if it cannot re-aquire the lock (rather than gracefully handling it).

The high frequency is easily explained since apt-daily runs both unattended-upgrades and aptd (via dbus triggers both from Update-Post-Invoke-Success (or whatever it was called) and directly from the apt-daily script) and this sets up a race with a fairly high chance of hitting unattended-upgrades somewhere where it hurts.

Even when it does not lead to a crash it often causes upgrades not to be installed. This can be seen by setting APT::Periodic::Verbose to 3 and monitoring the logs. Often one or another of the steps of apt-daily will fail because it could not acquire the lock.

Hope this helps.