To offer alternative views:

Some disorders develop over time, such as depression turning into anhedonia.

Some have comorbidities, such as anxiety and insomnia.

The individual may have been exposed to triggers, such as a psychedelic or stimulant bringing about bipolar or schizophrenic issues.

Medication or other drug use may have caused new disorders, such as stimulants causing anxiety and insomnia, or blood pressure medication causing lethargy.

A disorder may have been hiding behind the guise of another, like when ADHD symptoms occur due to sleep apnea, but the individual shows no other typical sleep apnea symptoms, so they're only wrongfully/partially diagnosed with ADHD and not the underlying cause.

Some disorders come with age, too.