The answer
Causes: Undersized duct supply/return runs; duct leakage (typical home loses 20%-30% conditioned air through attic duct leaks); lack of balancing dampers or zoning to counter natural stack effect; high solar heat gain on unshaded exterior walls/windows.
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.