Why do kids with autism regress?

Difficulty dealing: With stress or trauma they can have regressive behavior. Actually any child can regress in this situation but for an autistic child it may take less to disturb them. (did i get to the heart of your question?).

Kanner's 1943 paper : On autism described autistic regression in ~ 30% of toddlers, same as today. Neuroimaging of siblings of kids with autism shows atypical development of white matter at 6 -12 mos. & decline in gaze directed toward faces from 6-36 mos. In sibs who develop clinical autism up to 18 mos. After white matter changes appear, showing that autistic regression is far more gradual than previously thought.