Why does my door sometimes close itself?

Do you have windows open? It could be variations in pressure between inside your house and the outside.

Or ghosts.

My best explanation for this is that (probably, im not a scientist here) the different rooms change air pressure just slightly and as a result, to equal out the pressure you get a very subtle (or not) gust of wind between them (air flowing from one room to another). The slight breeze affects the door based on which way the breeze of going. This is off the top of my head, someone here will probably correct my logic :P