Chest splitting, shoulder arching gigantic slow long inhales!

Today (and several previous days) as I did my SONW practice, I experienced these huge inbreaths. At first, they seemed to want to fill my lungs beyond my capacity. This occurred during the hold in steps 4-5. They would fill up the lungs from top to bottom AND KEEP GOING, forcing the upper part of the lungs to expand and in the process pushing my shoulders back.

Later, the breaths were pressing the sternum forward.

I found myself with so much air to exhale, that I was exhaling through the mouth instead of the nose, even though I believe that both the inhale and exhale are supposed to be through the nose.