I found this in an old post from @EggcessiveTwo things that I could think of are blunt trauma injury to an air sac, especially in the neck or chest, causing air sacs to fill with air, and the chicken being unable to breathe. This is called subcutaneous emphysema, and it usually is contained in one area, but I have hear of severe cases where the whole body suddenly swells with air under the skin. The other thing would be swollen head syndrome which is a viral disease. In that, the head, neck, and wattles swell.
I was thinking some type of edema, but with the presence of HPAI, I completely understand your concern. I hope it's not the case. Please keep us updated