Not that this helps, but an educated guess for the cause would be the game glitching and registering two instances to get Aldner out of the cockpit that were both accepted at the same time. One tick later, and two Aldners are present on the ladder outside the cockpit. Their overlapping location probably resulted in the explosion forcing them away. As for a possible fix, maybe get both of them back into the same cockpit? Used to get rid of duplicate Kerbal's back before the proper implementation of crew...