Think ive done it myself a few times and seen some other people have the same issue, Probably a habit you want to get out of ASAP before it becomes too ingrained imo.
I found myself doing this when I was doing a MIG training course - breathing in just before pulling the trigger and then subconsciously holding my breath. Which then resulted in running out of breath, breathing in suddenly and thereby moving my shoulders and disturbing the torch position.
I created a sequence which I repeated over and over in my head as I welded - listing the things I needed to keep doing - it went something like:
Line - Travel - Angle - Stickout - Breathe
Line - was to keep the weld path moving correctly along the line of the joint
Travel - was to watch travel speed
Angle - was to keep the torch angle correct
Stickout - was to keep the distance from torch to weld pool correct
Breathe - daft as it sounds, this reminded me not to hold my breath and instead to breathe naturally
It stopped me holding my breath and I think it helped me to weld better.