In regards to the onset, the von Zumbusch form may "supervene on any previous pattern of psoriasis". It also may or may not recur periodically. Generalized pustular psoriasis of pregnancy (Impetigo herpetiformis)
This form of GPP tends to have symmetrical and grouped features. It usually onsets early in the third trimester of pregnancy, and generally persists until the child is born, but occasionally long after. In 2009, Dr. Debeeka Hazarika, president of the North East States branch of the Indian Association of Dermatologists, Venereologists and Leprologists (IADVL), published an article titled "Generalized pustular psoriasis of pregnancy successfully treated with cyclosporine" in Indian J Dermatol Venereol Leprol. As reported by Hazarika, there have been up to nine instances where the disease was recurrent in subsequent pregnancies. See Case Report 2
In 1979, Frank R. Murphy, MD and Lewis P. Stolman, MD reported on the case of a woman who developed generalized pustular psoriasis in response to doses of progestins, suggesting a link between progestogens and GPP. Most cases of GPP in pregnancy occur late in the third trimester, generally when production of progesterone increases. Infantile and juvenile
GPP is a rare disease in general, but even more so in children. In 2010, an article was published in Pediatric Dermatology by the Department of Dermatology, University of São Paulo. The report acknowledged that psoriasis is a relatively common skin condition in children, but "the pustular variant is rare." Out of 1,262 cases of psoriasis in children, a "0.6% rate of pustular variants" was found.
Through research and observation, many of these factors have been identified. The following table, from an article in Cutis, lists a few factors that have been observed as influential in the onset of GPP. Source: "Table II", "Pustular Psoriasis" Farber and Nall, 1993

Genetic factors
Although there are likely to be multiple genetic factors and environmental triggers, mutations causing defects in the IL-36RN, CARD14 and AP1S3 genes have been shown to cause GPP. Diagnosis
Classification
It is important to note that while there are different forms of GPP, they are not exclusive of each other. One can morph into another, or multiple forms can occur simultaneously. von Zumbusch acute generalized pustular psoriasis
Von Zumbusch psoriasis is named after the German dermatologist Leo Ritter von Zumbusch (1874–1940), son of Kaspar von Zumbusch, who described the first documented case of generalized pustular psoriasis in the early 1900s. See Case Report #1. Sometimes all or any of GPP is referred to as von Zumbusch psoriasis, but in the literature it is often distinguished as one specific form of GPP.Eugene M. Farber, MD and colleagues provide a description of von Zumbusch psoriasis in "Pustular Psoriasis", published in Cutis. They describe the pattern as having "waves of widespread or universally fiery redness". The affected areas are "painful and tender". Small sub-corneal pustules form, with sizes originally between 1 and 10 mm in diameter. These pustules may merge to form "yellow-green lakes of pus". The pustules dry out, and "Waves of scarlatiniform [resembling scarlet fever] peeling follow, removing the desiccating pustules".