If you edit the metadata for an issue, and the priority is not set for that issue, the form defaults to an actaul priority (i think the first in the list).
The form should default to the current set value, in this case '' otherwise the user will inadvertantly set the priority to something they don't want.
:thumbsup: it's been annoying me as well
Fixed in https://pagure.io/pagure/pull-request/949