This happens if the main hand weapon/tool is missing, because that determines the class you are switching to. We need labels for gear that is saved to a set, as well as the ability to make changes to a set without equipping it. I've had to delete and re-make three sets because of this issue. I probably wouldn't have deleted the items if they were marked as part of a set. Even if I had, editing the set without having to equip it would have been much more efficient.