Tangled Headphones Phenomenon

Do you know why headphones, Christmas tree lights, or any other items that are stringy knot themselves for no good reason?
If you buy a pack of headphones, you'll find that they are not tangled. But, if you put it in your pocket or in your suitcase, it will tangle in strange knots. Scientists have conducted tests on this by figuring out what knots form in this process and how often they occur.
Do you know why this happens?

                                               Image Screen-captured On murphed.com.