So it was 9 not 90 sec? In that case it really looks like hyperventilation (the purges are sufficient to lower your CO2 considerably). And it was in a warm environment? In plus sitting? Maybe you have even eaten or drank shortly before, or made a physical exercise? That all together sounds already pretty enough to trigger BO at a sensitive person.
The body reacts on the heat by dilating blood vessels to cool down quicker. That brings a blood pressure drop with the consequent slower supply of the brain. Normally, the body compensates dangerous pressure drop by constricting some arteries in limbs or body, but if you additionally (even just lightly) hyperventilated (what you did already by the purge breaths), then carotid arteries are constricted instead and the brain starves even more. If the digestive tract is active, because you've been eating or drinking shortly before, it gets worse. If you are sitting, it is worse, than if you are laying. Additionally, although you did not pack, you breathed to the max (limit of comfort), and then possibly relaxed (collapsed) to the chair, where the thorax and/or diaphragm certainly reduced the inner volume - the lungs pressed on the heart, reducing so the volume it can pump. That's the next factor reducing the blood supply of the brain.
Summing it all together, it explains the early blackout very well. I recommend reading the following document to understand the mechanism of fainting better:
Vasovagal syncope (fainting)