The National Psoriasis Association says that about ten percent of people who develop psoriasis have guttate psoriasis. A step (streptococcus) throat infection or another bacterial infection may trigger the disorder in susceptible people. Less commonly, other conditions may also cause the disease.
Like plaque psoriasis (the most common kind), guttate psoriasis is an autoimmune condition, though a trigger seems to be necessary to start the disease process. In some cases there also appears to be a genetic requirement.
According to the National Institutes of Health (NIH), some people with either plaque or guttate psoriasis develop psoriatic arthritis.