About 9 months ago, my 23-month old son began biting his sister when he became frustrated. Then, once he became more verbal the unwelcomed behavior faded away. Also, when I was able to figure out why he was mad, I would label what he was thinking, and that reduced the biting. So, if his older sister was playing with a toy he wanted, and I saw "that look" on his face, then I said "my turn" as I helped him point to himself and then had her share with him. So, in my son's situation the biting was due to him lacking the language skills to express what he wanted to say. Many of the kids I work with for therapy bite for the same reason, but others bite for other reasons. Here is a link to Zero to Three's website providing information on toddler biting and how to redirect it:
Zero To Three: Chew on This: Responding to Toddlers Who Bite
Out of the blue the behavior popped back into our lives again. Just yesterday, my son tried to bite my arm. This time it wasn't due to lacking language skills, it was because he was mad at me. He was playing at his train table, and I said "Let's go bye-bye". He didn't want to go, so I picked him up whereas usually he walks over to the door, and then that was when he attempted to bite me. But I caught him in time, and turned his face away from my arm. I then labeled his feelings and said "You are sad that we are leaving. Bye-bye trains and cars." He waved goodbye to his toys and then was happy again. My son is usually a happy-go-lucky toddler, but those "terrible two's" take over from time to time. But the nice thing he is usually redirectable or distracted easily. Us Moms and Dads have to be one step ahead of our little toddlers to help diminish those unwanted behaviors!
The Zero to Three link I provided above is not geared at children with developmentally delays or special needs, but I think the information still applies. Yet I also think there are other reasons for biting and undesirable behaviors with special needs kids including side effects to medication, seizures, and pain (from gastrointestinal discomfort, headaches, or other medical problems). Additionally, many of these children have multiple developmental delays which means a simple task such as completing a formboard puzzle or playing at the park takes more effort than it does for a typically developing child. By the end of the day, these young children are exhausted and it doesn't take much for a tantrum or "meltdown" to occur.
If your child receives occupational, physical, or speech therapy, be sure to discuss the undesirable behaviors and see if they have any suggestions.