The inflammation that causes the formation of pustules results from a disruption of the immune system which attacks its own skin cells. It is an autoimmune mechanism. 2
The course of the disease is unpredictable. You can have an isolated flare-up of psoriasis and never hear about it again or only present new lesions several years later. Most of the time, the symptoms reappear on the occasion of a stress, a change of life, a period of fatigue or an associated illness 1
References
1. DERMATO-INFO, Le Psoriasis - Consulted in January 2022
2. Pustular psoriasis: what is it and how to treat it? - Accessed January 2022