Leonbergers are one of the more gentle breeds, it's obscenely rare for someone to be attacked and killed by a breed like that.
My guess is, due to eight dogs being walked at the same time, something just went haywire and pack-mentality kicked in, and whoever was there, was getting killed.
I can't imagine it really being possible, to walk not only eight dogs at the same time - but dogs of all different shapes/sizes/breeds.. It sounds like an absolute ******* nightmare.
A friend of mine has 5x Akitas, all exactly the same and he walks them at once - but all the dogs grew up, live and play together, and he's owned Akitas all his life and has never had a problem, he's also knows how to impose himself as the leader, I've never seen his dogs even get close to being out of control.
This whole story just sounds like someone trying to do far too much at once...