From what I know of Jason Russell, the sudden, whirlwind and emotional and behavioral demands that KONY 2012 is more than enough to be a stressor to trigger this disorder. Psychosis means that one has a break from reality and may have delusions (having odd thoughts that one believes are real) and hallucinations (hearing, seeing, feeling, tasting, smelling, etc things that no one else experiences). It is marked by poor judgement, cognitive impairment or acting on the basis of a delusion or hallucination.
As with all mental illness and disorders, they are very real and not an excuse for inappropriate behavior. In this case, he did not have the resources to deal with his mounting stress. It is not a character flaw and does not mean that he cannot continue to make fantastic work. It does mean that he needs a support system and resources to manage his stress levels.
The following is from http://www.sabp.nhs.uk/eiip/index_html/client/what-is-psychosis :
What is Psychosis?
The symptoms of psychosis vary a lot between individuals and not everyone will experience those that we have listed.