I came off at Cadwell too last year (was at Chris Curve tho). Someone else on the site recomended F30 R29 for tyre pressures and these worked pretty well for me at the last trackday i did.
As for why you fell off... dunno mate.
I still don't know why i fell either. I wasn't cornering as hard as i had done earlier in the day, maybe just went off my normal line slightly and found a bump. Like you say, maybe just one of those things.