This is actually a diagnostic criteria for autism called “echolalia”. Both verbal and non-verbal children may do it. It seems odd coming from a child that doesn’t speak otherwise because it’s obvious they have the ability to speak but won’t speak on their own in their own words. Some children with echolalia are so good at it that they can match voice pitch and words exactly, duplicating exactly what he or she heard.
With verbal autistic kids, they are trying to remember, verbatim, everything they just heard in a commercial or short video about something they want or that interests them. It may even become quite obsessive, repeating the exact same thing every time that one subject comes up. It’s something they just can’t help doing.
Acknowledging that you heard them may stop the echolalia for a short time, or they may continue to repeat what they saw and heard as a means to get you to buy the object of their desires and not forget exactly what it was they wanted. Reinforcing the behavior by buying that item for them means it will continue even after they have the item they talked non-stop about.
One other thing parents need to consider is that children with autism, verbal or not, will use echolalia to try and relate things they might have witnessed. Trauma will cause a child with autism to repeat what they saw and heard over and over again, like a broken record or home movie of the events. Paying close attention to the echolalia at this point will really help others make sense of what happened.
The other hypothesized reason that children with autism repeat words and phrases is that they like the sound of what they repeat. Most languages have a natural rhythm or syncopated beat. Most average people don’t pay any attention at all to the up and down beats of the words and sentences they are saying. However, children with autism are cued in to musical rhythm, which mimics language rhythm. Because of this, it’s thought that they hone in on the language rhythms created by words and phrases and like repeating them for the musical movement they seem to have.
At any rate, it’s still a compulsory thing, and understanding the causes and reasons behind it is as mysterious as what the non-verbal autistic child is thinking.
Check out these questions as well.
1). Why do so many autistic kids cover their eyes/face/ears with their hands?
2). Why do many kids with autism flap their hands?