It may be that these children were already genetically prone to OCD or tic disorders. It seems to occur more in children before puberty. Characteristically, there is a sudden, “overnight” onset of symptoms. Besides classic obsessive-compulsive behaviors or tics, children with PANDAS may exhibit choreiform movements, irritability, hyperactivity, severe nightmares, separation anxiety, daytime urinary urgency without associated UTI signs, and age regression.

The mechanism of action is unknown but may be related to autoimmune antibodies or a toxin affecting the basal ganglia. Treatment of the GABHS infection seems to result in a decrease or resolution of symptoms for most. Exacerbations of symptoms may be from re-infections with GABHS (which can be subclinical).

I could not find any good data about the incidence/prevalence of PANDAS.