We would use Ref +20 back in the airnet days for circling. That speed would all for a small safety margin for turns in the pattern (usually about 20 degrees of bank max when configured). Wx at the time of the crash was gusty, if the crew got low and slow and then combined with an unpredictable gust, could be the perfect setup for a stall/spin event.
Prayers for the victims and their families....