Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to examine your dad and obtain a history and review the imaging, I can not tell you what the exact cause of the symptoms is. However I will try to provide you with some useful information.
MSA-P, or multi-system atrophy-Parkinsonism type, is what is termed a parkinson's plus syndrome. Basically, there are some features of Parkinson's disease, but there are additional features, such as cerebellar ataxia. Ataxia is basically a constellation of physical examination findings such as unsteadiness, wide-based gait, inability to coordinate movements etc. The cerebellum is part of the brain at the posterior aspect, and is involved in gait steadiness among several other functions. The diagnosis is really a clinical one (based on examination) but can be supported by MRI findings. There is unfortunately not a test that can definitively make the diagnosis of MSA, and there is no cure. Treatment is symptomatic, treat the symptoms, but not the underlying disorder. Some patients respond to the treatments for parkinson's disease, such as levodopa, for a time. Physical therapy for gait is another important management component, as is assessment of swallowing to ensure that aspiration leading to pneumonia does not occur.
There are several causes of cerbellar atrophy, including but not limited to excessive alcohol intake, some medications, some genetic disorders, such metabolic/genetic disorders, some vitamin deficiencies, and others. These are usually clinically distinguished, with support by laboratory findings in some cases.
Evaluation by a movement disorders specialist (a neurologist specialized in movement disorders) may be of benefit.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.