"Cause? Made worse, perhaps," Snape drawled. "My guess would be that the location of the Door in France has always existed as a...magical counterpart to the Veil. When your magic reached out to Sirius, even the first few tendrils, the balance in the world beyond began to fracture. Places outside of our realm do not take so well to intrusion, even of the magical variety. Since the Veil is contained in the Ministry, with countless protection spells surrounding it, it remained unchanged. Meanwhile, the Door, out in the open, was able to unlock. That, or these paint fumes turned me mad," he added with a sly smile. "Whatever the case may be, I reckon that the Door was bound to break open eventually. You just found yourself, once again, at the wrong place at the wrong time. Troll in the toilets all over again."