Also, it could be because a thread you had responded on got deleted, and it had nothing to do with your particular post. The whole thread got chopped.
Most people who post here very much have been deleted at one time or another. Not a big deal. If you know it was specifically your post that was deleted, and you are curious why, contact admin privately. They won't discuss it on the board.
If a post get political it will get deleted. The Admin are fairly tolerant of political posts as long as it strongly relates to religion in general and mormonism in particular but if it doesn't you can safely bet that it will be deleted.
Another common reason a thread goes missing is because of trolling. It might not have been a troll starting the original post at all, it can be a perfectly legitimate post with lots of legitimate responses, but it's sufficient that a troll stirs up a bunch of trouble in the thread and it will go poof.
Other than that I've had threads disappear without the slightest clue why it happened. When this happens you can always send an email to Susan if you think it's worth the trouble.