What are the possible ways a child can become autistic when it doesn't run in either parents family?

Cause undetermined. The cause of autism has not been definitely defined. While it does tend to run in families, many autistic children are born to families without a history of autism. While there may be a delay in presentation, the cause is determined during fetal life and possibly affected by environmental factors.
Genetic alterations. Some kids having the ASD or autism label have had very refined chromosomal mapping and assessments done. Many have small areas of redundant genetic material added or missing from otherwise normal appearing chromosomes. Families with more than one symptomatic child seem to have this more frequently. This points to events that altered the genes in the egg prior to conception in some cases.
Another possibility . Has been studied at the mind institute in animal models & has now been reported in one study in humans: fetal exposure to certain "epigenetic factors" which change the function, but not the structure of fetal genes, (e.g., "the flu" & other infections that cause maternal fever > 1 week, environmental toxins & certain drugs) increases the risk of autism. More research is needed.