As discussed on T209802, GPG 2.1+ automatically starts a gpg-agent child process and leaves it running after the parent gpg exits. This does not suit SecurePoll's pattern of use of gpg. At best, it is a resource leak. If MediaWiki is configured to use cgroup-based memory limiting, ($wgShellCgroup) a background monitor process will wait forever, expecting gpg-agent to exit so that the cgroup can be cleaned up. If HHVM LightProcess is used, the PHP script also hangs waiting for the background processes to exit. Perhaps there are also security implications, if the gpg-agent inappropriately persists the private signing key.
The workaround for now is to install GPG 1.x and to configure SecurePoll to use it with $wgSecurePollGPGCommand.
@faidon suggests migrating to a GPGME wrapper such as http://php.net/manual/en/book.gnupg.php