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Sometimes when they suddenly start acting aggressive it could mean they don't feel well. A friend of mine has a pionus who was very sweet, one day she started lunging at her and it was the only sign she was very ill.
Aggression can also hormonal related, puberty, not sleeping well at night, they are just in a bad mood that day(everyone has their off days), or a change in their environments, their humans do something they do not like. Sometimes the smallest changes can make them unhappy and we have to try hard to find what it is.