What causes the mind to creat radom thoughts?

i have notisd that i can be doing just about any thing and negative thoughts and memories op in to my head. the other day i was changing my nephews dipper and the image of a car wreak pop in my hear for some unknown reason. what does a little boy and a wreak have to do with each other!other times i can be walking down the street or looking at the computer and my mind will bring u images of people dieing, my ex girl friend, shooting my self with a nail gun. some of these thoughts can be kind of disturbing. the strange thing is that i am not angery and the only stress i have is from my college cles.