I agree with paxiled questions and am interested in those things too. There are, really, worst thins to have a compulsion to do. You are essentially slowing your heart rate down and getting more oxygen into your system then. I encourage my son with anxiety to have periods of the day in which he does the deep breathing you describe. So, while I'd be curious why it is happening, again, it's not a dangerous thing to be doing and actually could have some benefit. Unless you can't breath before hand from exertion . . . that needs a doctor consultation.
Any idea if something happened before you did this the first time? Are you in therapy to try and fix this or just maintaining on medication? Do you do any relaxation exercises, such as meditation or breathing exercises?