I think you might mean that someone added a new option to the list of priorities? So that people editing or creating issues can now select what seems to be quite a silly looking priority (if it's a 1, surely it should be "critical", not "medium"? Or "trivial" if you order things in the opposite way)
If I am right, then this change was made by an admin, and may have been caught in the "administration audit log" (go to admin and search for "audit", that should find it for you).
If it has not caught it, I'm afraid all you can do is ask all your admins.
I'd be very tempted to rename it as something mildly annoying like "nonsense priority" and see who yells!
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