It doesn't mean anything. It means it needs to be straightened (or if so aesthetically inclined, leave it the way it is.) If you're thinking of something "spooky", I doubt there is any such explanation to such a phenomenon unless you are witnessing some active anomaly right before your very eyes, and even that might mean you have ingested some serious psychedelics and/or some serious quantities of such. If you are looking at the cross, and you know you have not ingested some questionable substance and the thing flies off the wall into your face, drawing blood, or something then you may indeed have something "spooky" going on.
It would be hard to determine anything from finding your cross tilted. Possibilities are that it was somehow lose and tilted by the effect of the wind or someone might have accidentally tilted. Otherwise, it is best to assume that no one was behind it until you get them red-handed.
My pictures get tilted every now and then. Could be caused by vibrations, either from doors closing, you walking, or cars going by. Maybe from the ceiling fan breeze, or rush of air when you closed your door. Just push it up a tad, and it will be good as before. =)