Usando a sugestão de @Seth, descobri que é porque eu instalei o dunst, um daemon de notificação dmenu-ish.
dpkg -l | grep noti
ii dunst 1.0.0-2 amd64 dmenu-ish notification-daemon
ii dzen2 0.9.5~svn271-3 amd64 General-purpose messaging and notification program for X11
ii freespacenotifier 4:4.11.5-0ubuntu0.1 amd64 free space notification module for KDE
ii gir1.2-notify-0.7 0.7.6-1ubuntu1 amd64 sends desktop notifications to a notification daemon (Introspection files)
ii inotify-tools 3.14-1 amd64 command-line programs providing a simple interface to inotify
ii kubuntu-notification-helper 13.10ubuntu1 amd64 Kubuntu system notification helper
ii libevent-2.0-5:amd64 2.0.21-stable-1 amd64 Asynchronous event notification library
ii libgtk2-notify-perl 0.05-3build1 amd64 Perl interface to libnotify
ii libinotifytools0 3.14-1 amd64 utility wrapper around inotify
ii libknotifyconfig4 4:4.11.5-0ubuntu0.1 amd64 library for configuring KDE Notifications
ii libnotify-bin 0.7.6-1ubuntu1 amd64 sends desktop notifications to a notification daemon (Utilities)
ii libnotify4:amd64 0.7.6-1ubuntu1 amd64 sends desktop notifications to a notification daemon
ii libstartup-notification0:amd64 0.12-3 amd64 library for program launch feedback (shared library)
ii libstartup-notification0-dev:amd64 0.12-3 amd64 library for program launch feedback (development headers)
ii libzephyr4:amd64 3.1~beta0-1 amd64 Project Athena's notification service - non-Kerberos libraries
ii muon-notifier 2.1.3-0ubuntu0.2 amd64 update notifier for KDE
ii pidgin-libnotify 0.14-9ubuntu2 amd64 display notification bubbles in pidgin
ii python-notify 0.1.1-3ubuntu1 amd64 Python bindings for libnotify
ii python-pyinotify 0.9.3-2 all simple Linux inotify Python bindings
ii update-notifier 0.147.1 amd64 Daemon which notifies about package updates
ii update-notifier-common 0.147.1 all Files shared between update-notifier and other packages
ii vlc-plugin-notify 2.0.8-1 amd64 LibNotify plugin for VLC
ii xfce4-notifyd 0.2.4-2 amd64 simple, visually-appealing notification daemon for Xfce