Sycamores are indeed trashy trees and their root system is weak if I understand correctly. They are huge beasities and that kind of tree often rots from the inside out. If the root system is compromised by the core wood being dead it can not support several thousand pounds. It appears they were at a commercial campsite. No way is a reputable campsite going to leave an obviously bad tree in a place where it could land on a tent site. I'll bet you could have looked at that tree top to bottom and not seen anything to give you pause. It just let loose.