Intruder alert!!

I have a question. Where -do- intrusive thoughts come from?

Seriously, are they random outbursts of some unknown facet of our minds? Are they random possibilities simply making themselves known? Are they simply subconscious urges? and if so, does that mean that the subconscious mind of those who experience intrusive thoughts on the regular is somehow stronger than their rational mind? (in relative relation, of course. we know that the subconcious is more powerful than reason in certain ways. But is this strength somehow enhanced in certain people?) or, does it instead indicate that said person has a deeper connection between and therefore, awareness of, both?