This behavior is something I notice with my macaws, not so much my medium to small sized parrots but definitely with my macaws. If I offer my hand to them straight up to their bodies sometimes they will get put off and angry and lunge if I don't read the body language well enough telling me they'd rather not come in the first place. I also notice it if they are higher up than me in the aviary or if another bird is irritating them (usually my blue throat macaw Jinx gets the blame for being the annoying brother!) However, if I offer for my macaws to step back rather than step up, they will 10 out of 10 times do it. Maybe it's less invasive of their space, maybe they feel less challenged, maybe it's something else entirely... I'm not much of a "why" person if you haven't gathered that yet, and so I gladly take it as it is. I try not to over-use it but if I need to wake my birds up in the middle of the night in the aviaries to get them inside before it gets too cold, this is the method I use. The step back instead of the step up.
Article by Jamieleigh Womach. She has been working with parrots and toucans since the age of 17. She isn’t homeless but is home less than she prefers to be. She travels the world with her husband, daughter, and a flockful of parrots whom she shares the stage with.