PDA is pathological demand avoidance. It comes with some types of autism or with PTSD, autism in our case (likely three of the four of us, but it's not always diagnosed on its own in the UK). It's worse with anxiety, so stressful situations really set it off badly.
It looks from a distance like defiance, but it's a full on fear response to almost any demand, like being asked or told to do something, even if it's something they'd otherwise happily do, leading them to utter (sometimes frightening or damaging) extremes to avoid the issue.
DD used to sabotage all her own birthday treats, excursions, refuse to do as her dad told her - even if it was clearly to her benefit. She'd literally hurt herself to avoid complying. And she looked shit scared the whole time.
In adults it often manifests as an inability to bring yourself to open letters (so debt builds up), to clean according to a schedule (can do chores on impulse but not to routine). Even routine hygiene can be an issue when it feels like a demand.
It's awful, and the sufferer is generally very miserable. Especially if they don't understand why they're like this. She knocked her own tooth out once to avoid the tenseness of waiting for the tooth fairy. She stopped eating or drinking for three days when I tried to get her to eat healthily - she was very close to hospitalisation - she was only 3.