What are some possible causes for a thread to vanish?

Mods, mostly.

Can't the originator also delete it?

Yes. A permission that I strongly oppose. After enough posts go in there, the thread becomes part of the forum, not simply a post by the OP.

Well it's nice when you start a thread and someone lets you know there already is a thread on it. Instead of a mod having to delete it or lock it, you can just delete it yourself and then post in the already created thread.

Well, I posted this morning, asking for opinions on my coloring book idea, and the thread is gone.
Here's the sample pictures again:

I was wondering if anyone thought that a 20 picture book like this would sell for $9.75? Or if I should save my money and get something different printed, like a deck of Tarot cards.

Mrs. Danger:

I can't check on this, because I am at work, but looking through your post history I think your thread may have been moved to the Adult Forum (if it wasn't started there to begin with).

Those are not the same pictures.

For that matter, what the hell happened to Julie Walker's thread about women in Muslim countries? I posted something and when I came back here the thread seems to have vanished. It wasn't because of my post, was it?

Seems like the OP deleted it.

It would be nice to get a note... so I don't make the same mistake again.

Ack! Naked unicorns! Moving to adult.

They're making me horny.