The easy part of this answer is that the breathing a certain way is a compulsive behavior that he performs to try to undo an obsessive thought or image about you being harmed.
The more difficult part of the answer involves trying to help you understand why he does this.
To get an idea of what a breathing ritual can look like if you’ve never seen one, try to watch the MTV show True Life: I Have OCD (2005). On that show is a male who does a lot of breathing rituals, in addition to many other compulsive behaviors.
Let’s think about some possible reasons for the breathing ritual. It is possible that your husband once had an intrusive thought about you being harmed as he was yawning. That feeling of not breathing when yawning could have been paired with a thought of you being harmed and it caused him more anxiety than the thought alone would have. Now he wants to be sure that he breathes a certain way to be sure that no harm will come to you, as well as to prevent himself from experiencing the anxiety associated with not breathing.
Or maybe one day he saw you almost get hit by a car, and as you were about to get harmed, he took a deep breath (as would be common), and just as he did that, the car swerved. He may have paired in his head the breathing and the car swerving, so he feels that he needs to continue to breathe that way to keep you safe, even if he’s combating only a thought about something bad happening to you.
Whatever the reason for them, the breathing rituals will probably seem very strange to you, and in a way that is good because it means you can help your husband see the irrationality of the thought.
As strange as it sounds, his motivation really is to keep you safe, but as noble as his intentions may be, his compulsions are also extremely unnecessary, given that his breathing and your safety obviously have nothing to do with each other. And this is exactly what he will have to learn through therapy. You will probably not be able to just convince him to stop.
Find a therapist who can assist both of you with this; it may be necessary to include you in the therapy so that your husband can see firsthand that not doing his rituals does not lead to any harm befalling you.