Interestingly enough, in many cases it did not appear to be the actual stress of the earthquake that caused the plucking. The continual stress of aftershocks was the factor to many birds. Parrots have encapsulated nerve bundles in their leg joints that seem to act as vibration detectors. Consequently they are highly sensitive to vibration. I know of parrots who started plucking when their owners moved to downstairs apartments or next to a freeway.
Parrots are also highly empathic picking up their owner's energy in times of stress.
Allergies, including food allergies, household dust, and seasonal allergies to pollen or mold spores may be a cause of picking in some birds. Food allergies may also be a significant factor. I personally question the use of some food additives, particularly artificial food coloring, in the manufacture of pelleted diets and won't feed them to my birds. Some birds may have negative physical reactions to components in other foods and dietary supplements. It is however, difficult to pinpoint the exact cause of food related allergies.