Question of the week

How do light bulbs unscrew themselves? This can't be only be happening to me. My lamps are really old and rickety for the most part but I don't think that's the reason that I have to retighten the bulbs every once in while. Sometimes I think the bulb is out and discover it's just not twisted in far enough. And today I changed a bulb that did burn out but it was only one twist away from unscrewing itself and leaping to the floor.

Are my lightbulbs just suicidal or is there a scientific explanation for this do you think?