When I started advocating for mental health issues, I was adamant in telling people my mental health issues were not caused by my disability. I didn't want to make mental health another part of my disability. I thought this would further ostracize people with disabilities in the mental health community.
Now I am not so sure the issues are separate. My mental health issues may not be directly caused by my cerebral palsy, but they definitely factor in. Sometimes I feel very isolated. I volunteer from home and sometimes it is hard to go out. I get tired easily at night. Many places are, unfortunately, not accessible. I may find an event online I would like to go to only to find out there are stairs or the washroom can't fit a wheelchair. Sometimes, when I am not sure if the venue is accessible, I could call or email to ask, but I am afraid they will say they aren't accessible or I just feel blah and I don't want to put out the effort. I know that feeling is part of the discouragement and depression. It's a downward spiral.
When a person with a disability has a secondary health concern, its not a matter of one being the cause of the other. It's impossible to tell where one ends and the other begins. We have to see the whole person with both joys and challenges.
By the way, this is a really interesting study about mood disorders in Canada. It speaks to the prevalence of different mental health issues among various groups of people. Sadly, people with physical disabilities aren't even mentioned. That is very telling, actually. We are so often an ignored demographic.