At first thought my answer is inflammation.  Let us pursue the answer and see if I am right or wrong. COPD and Arthritis: What’s the Connection? (creakyjoints.org)  Creakyjoints.org seems to agree with me with another caveat about the connection being cigarette smoking.

COPD — short for chronic obstructive pulmonary disease — is a lung condition that makes it difficult to breathe. Rheumatoid arthritis (RA) and other forms of inflammatory arthritis are joint diseases that make it difficult to move.

“So, what does one have to do the other? More than you might think. Research has shown people with inflammatory forms of arthritis are likely to develop at least one chronic comorbid disorder — and COPD ranks among the highest-risk diseases for patients, compared to those without inflammatory arthritis. In fact, a study published in the journal Arthritis Care & Research suggests people with rheumatoid arthritis are nearly twice as likely to be hospitalized for COPD than the general population. Other research suggests there is an association between ankylosing spondylitis and COPD.”

The Centers for Disease Control (CDC) “study demonstrated that the prevalence of COPD was almost 50% higher among adults with arthritis than among adults without arthritis in the United States.”

West Park Healthcare Center  COPD and Osteoarthritis | West Park Healthcare Centre finds that people with COPD are 3 times more likely to have osteoarthritis.

As you can see, I am finding that there is a connection between COPD and arthritis, but find truly little explanation for why, so I resort to Google Scholar for more information.  I found an article in The American Journal of Respiratory and Critical Care that states “The common link between COPD, asthma, and chronic arthritis is the presence of a chronic inflammatory state. Accordingly, the fact that patients with all these diseases shared an increased activity of Cytochrome Oxidase (CytOx) in their circulating lymphocytes suggests that, somehow, this may be linked to chronic inflammation. Whether or not this is a marker of disease activity is presently unknown.”