The owner of the coach company has said it is thought the driver had a heart attack but this is unconfirmed, he was 63.
To be fair, this might be 'damage limitation'. Until the post mortem is done it's just as likely that he was half asleep, hence leaving the road on a gentle curve. If the driver had keeled over at the wheel it's unlikely he'd have continued on the straight, though of course it could all have happened just as he reached the bend. Speculation doesn't help though, perhaps we'll hear in due course.
With regard to the capacity issue, if indeed the coach did have 53 seats in total then it sounds like the coppers simply got it wrong and the press have got over excited. Chances are that somebody went to check on the seating capacity signage and assumed +1 for the driver, and just didn't realise there was ever a crew seat. With the damage to the coach it probably wouldn't have been obvious. Every press report screaming 'overloaded' had me thinking he was plodding up the A3 with a standing load, typical media rubbish.