I wonder if it due to reduced or increased blood oxygen. Certainly hypoxia can cause a variety of symptoms such as anxiety and panic. Did you measure your blood oxygen at the time?
Presumably if the breathing exercises are to be useful, the user should be getting the normal amount of oxygen. The problem I see is that if you're actively controlling your breathing, you're bypassing the normal automatic breathing, so it is probably easy to get too much or too little.
The only time I've deliberately used breathing exercises was to see if I was able to increase my blood O2 levels above 12000ft, and I did manage to successfully increase it from about 80% to 90+% using deliberate deep breathing.