In breeding pairs of some cockatoos, this may appear as mate aggression. It is extremely important for one bird to feel that she, or he, can retreat to a safe distance if necessary, from any threat. For example, during breeding and nesting season in the wild, a female can leave the nest, tree, etc. flying to a neutral space if she feels threatened or uncomfortable, due to the males assertive hormonal behavior. In a cage or aviary, this can become a totally different situation. I know personally of a number of instances where 'aggression' from a male has resulted in serious injury, such as the beak being completely torn off the female, and even death.
Some birds also seem to do a great deal of gesturing prior to resorting to a physical attack.
One of the most common postures when feeling threatened is for the male to 'stand tall', holding one of his feet up and pushing it toward the female, often hissing at the same time. There are also quick pecks with the beak in an effort to encourage taking flight. If we look for just a moment at several examples with differing outcomes, let's say I'm playing hide and seek on the floor with my pet corella, and that this is something we do every day. We've been hiding around corners, chasing each other around the chair and suddenly the bird takes off down a long hallway, stopping at the end because there is nowhere else to go. If I keep running in the same playful manner and reach down to pick up this 'cornered' bird who is crouching down, looking up at me and hissing, there is the possibility my action might be perceived as a threat or danger.