Almost every single thing that happens to people with psoriasis is unpredictable. This brand of psoriasis erupts rapidly and profusely in small drop-like scaling plaques all over the person. It does not matter if a person's throat was sore or not. The bacteria were there. Naturally, that is where the mystery starts.
Nobody has any idea what it is about the infection that produces the spots.
Another unanswerable issue is whether or not a guttate flare will happen each time that person gets a Strep infection. When this association was realized some decades ago, attempts were eagerly made to see if all forms of what is called psoriasis were somehow caused by Strep bacteria. Treatments for guttate psoriasis are no different from or better than the multitude of options for common psoriasis. But some people forget to read the books. Their cases can be as legendarily resistant as any psoriasis.