Probably they thought, if they really going to have a problem with the doors, then they will reach a lower level a bit faster? And probably being under the majority of traffic on a really busy airway, they have to be concerned less about dealing with other airplanes on the way down, which might have been on weather avoidance? Etc, Etc :
Obviously I don't know all the details what led to the decision of the KA Crew to descent a bit lower but neither do you. But I am sure they had a more complete picture than you and I or some Monday morning aviation quarterback here on pprune
