If you Google the history of the zentangle, you really won't find anything that specific on the subject because it is something that defies a tidy definition. When you look at an example, at first glance, it may just appear to be a bunch of doodles.
But, it is more than that. Some practitioners believe that it serves as a form of meditation. There is some deliberate thought involved initially but they easily fall into a rhythmic, relaxed state as they draw the different designs.