If installing and or removing packages is interrupted, the system can be left in a bad state. Yum should take a systemd shutdown inhibitor delay lock (see http://www.freedesktop.org/wiki/Software/systemd/inhibit) when doing anything that can't safely be interrupted.
Should be in tomorrow's rawhide, -151. http://koji.fedoraproject.org/koji/buildinfo?buildID=538517 ...rebuilds in the copr and my fp.o repos.
Just FWIW, rpm >= 4.12 (once it hits rawhide, sooner than later now...) has a plugin to do this on rpm level.
yum-3.4.3-152.fc20 has been submitted as an update for Fedora 20. https://admin.fedoraproject.org/updates/yum-3.4.3-152.fc20
yum-3.4.3-152.fc20 has been pushed to the Fedora 20 stable repository. If problems still persist, please make note of it in this bug report.
yum-3.4.3-152.fc19 has been submitted as an update for Fedora 19. https://admin.fedoraproject.org/updates/yum-3.4.3-152.fc19