Any and all behavior change should be viewed as a possible communication tool.
For some people with Down syndrome (DS), like some without DS, a behavior change may be a form of communication. This is probably more common for people with DS because some have a limited ability or no ability to use spoken words to communicate (verbal expressive language). Even some people with and without DS who have good verbal expressive skills may have difficulty verbally conveying emotional, physical, or social stressors, so they present as a behavior change.
What might the behavior change be communicating? It might be a physical health problem or an emotional or social stressor. A painful condition such as a sinus infection or gout may present as irritability or agitated behavior. A change in the boss at work may present with a withdrawn or sullen mood. An illness of a family member may present with less tolerance of the activity of a young niece or nephew. A wide variety of conditions can contribute to a wide variety of behavior changes.
When evaluating a behavior change, if no assessment is done for these underlying conditions, it is likely there will be an incomplete understanding of the individual. If these underlying issues or conditions are not addressed, the treatment will be incomplete and less likely to be successful.
Resources
Impact of Physical Health Conditions on Mental Health (article)
Behavior Changes in Individuals with Down Syndrome (webinar recording)
Assessing Physical Health/Mental Health Connection (Chapter 2 of Mental Wellness in Adults with Down Syndrome)