How do preference files become corrupted?
It can be caused by a great number of things, but I believe the most common cause is where the file is being written to, and the application terminates before it's finished writing the file. This could be caused by system overheads being too great for a successful write at the time.

Hardware errors with the hard disk, or memory could also cause data errors, but I would imagine these would be the least of your problems by this time.