Doors have hinges. Hinges are a movable joint which a door swings on. Sometimes with repeated use and age, a door can become unhinged and will need to be fixed. To "become unhinged" can also refer to a mentally unbalanced, or deranged human being. The joke is referring to old door framers, and is playing on the word "unhinged". 
OLD DOOR FRAMERS never die , they just become unhinged .