The answer
Motor inrush surge causes momentary voltage drops when heavy compressor motors start (4-6x running amperage). Normal if split-second; abnormal if flickering persists, affects multiple rooms, or causes simultaneous dimming/brightening (indicates undersized feeder, overloaded circuit, or loose main neutral).
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.