I can’t vouch for why you are yelling at this moment, but I know how you feel. You are trying to be heard. She can't hear. If you are a daughter, it is a scientific fact that raising your high-pitched voice won’t get you heard if your mother is old enough to have sensory-neural hearing loss in the high frequency range. If you are a son, well she might be more likely to tune you in.
You might be yelling because for all these years she has never listened. All these years later even with good hearing, she doesn't understand language anymore. That is receptive aphasia even if its not a named diagnosis yeta.
You are yelling because you’re busy and she isn't moving fast enough. You are on a schedule to get many quantities of things done today and your mother doesn't have a schedule, but likes being on yours. You misallocated her allotted time. You are distracted and not engaged with what is important to her. You take for granted that putting on shoes or a coat consumes less than a minute. She is just happy to be going to the dentist and for each step of the task she is in the moment, practicing her Buddhism. Your mother has less time remaining so she is here in the now and you too are here with her in her now. That’s why you are yelling.
Your mother is yelling at you. I have no idea why, but you are hearing it and feeling it loud and clear. She had occasion to yell at you 30 or 40 or 50 years ago. Plus ca change, plus c’est la meme chose. The years did not change the dynamics except to make it painfully clear to your mother that maybe you don't think she is in charge anymore.
You are distracted and not engaged with what is important to her. You don’t understand that she doesn't understand. She doesn't think she needs your help. She doesn't know that she needs your help. You are yelling because after all these years you thought she would have wanted to listen.
Try this: Google “my mother doesn't listen to me” 62 million hits, suggesting a wide spread problem.
Listen: To give attention with the ear. To pay attention heed; obey.
Antonyms: ignore, tune out
sensory aphasia noun: inability to understand spoken, written, or tactile speech symbols that results from damage (as by a brain lesion) to an area of the brain (as Wernicke's area) concerned with language—called also receptive aphasia, Wernicke's aphasia