"There are many children in mainstream schools with SNs, SEBD are not in the majority."
Many children with special needs have explemorary behaviour. One of the most common groups are children with moderate learning difficulties or children with dyslexia. Many children with autism are well behaved as well.
ADHD is another condition that is poorly understood.
"I do absolutely believe that there are children who genuinely have special needs. However, is there some medical reason why there are a disproportionate amount that come from from homes where there are parenting issues?"
Genetics, pure and simple. Parents with learning difficulties often have children with learning difficulties. A parent with low intelligence will struggle to access help to or the common sense to parent to a high standard.
A lot depends on what your definition of parenting issues is. Someone with major learning difficulits may well try their best to parent their children. The children will not be abused, but might be fed a crap diet, never have their teeth brushed (because parent doesn't brush their teeth), never read to because the parent can't read and have no routine.
There is a difference between abusive and sub optimal parenting.