How mom can deal with dad's sudden bursts of anger toward her?

He is in great health per recent ckup.

In-laws in good health, mid-80's. Ma has bad scoliosis but still mobile and understandably slower in past couple of years. Dad is always active and stays busy.

She's becoming more stressed over dad's sudden bursts of vocal anger and at her and increased grumpiness. Says she can't deal with his behavior. Believes medication to be the answer. Otherwise, they get along great.

MD recently proclaimed him in excellent health; very active; prides himself on needing no medication. We prefaced his appt with letter to MD detailing mom's concerns.

Otherwise, they're great together. He's helpful with household chores, takes her places, etc.

Ma has never challenged him. He says he will try to avoid outbursts and it works for a while. He's unaware that he is upsetting her so much. Have advised her to be more open, but not her way. When witnessed by son and told to STOP IT, he eventually will but one can tell he does not feel he is at fault.

I'm thinking this is due to her declining health and his increased sense of responsibility which is stressing both of them so much that they can't see it and don't know how to resolve it correctly? Seeking advise.