It's puberty, the point in which they become sexually mature and get a surge of hormones. All parrots have it happen, it's just like when kids go from sweet little children into teenagers. A lot on how badly the react has to do with how YOU bond with them, train them and what you allow them to get away with. One thing you must accept though, when owning a parrot, is the fact they all bite at some point. They are not feathered dogs, they are essentially wild animals, as they are not enough generations out of the wild to be truly domesticated (i.e. only positive traits bred down through thousands of generations). I have one of the most notoriously 'vicious' species (BFA, one of the hot 3 zons) when it comes to hormones, and he's a male to boot. It's not THAT big of a deal because we handle it appropriately and don't take the occasional lashing out personally. The positives of having our generally sweet, loving, inquisitive, friendly bird FAR outweigh the occasional nip or aggressive display due to natural hormonal fluctuations. Plus there are numerous things you can do to prevent or reduce aggression.
When it comes to aggressiveness with our bird:
He is NOT allowed to be vicious (and is corrected if he is) and he is NOT allowed to display 'mating' behaviors (and is ignored when he does). He IS stick trained for times he is too unpredictable to handle and we DO take as many preventative measures to reduce hormonal aggression as we can (such as keeping a year round, consistent sleep/awake schedule, feeding more vegetables and less sugary fruit during his mating season, consistent reinforcing positive behavior/ignoring negatives ext...). You will also learn your birds body language, as they usually display some signs they are feeling aggressive and should be left alone (not sure what caiques do, but pinning eyes, flared tail, slightly open wings are all pretty universal indicators of a PO'd bird).
If you mess with a bird clearly communicating he or she wants to be left alone, you can't blame the bird when you get bit. Plenty of people do though. Same with having no clue why the "bird just randomly turned vicious" at a certain age because they never did proper research and have no idea it's hormones or how to handle them. And those 2 points are where many of these parrot "horror stories" stem from.