One of the philosophies that I apply is that "if it isn't on you, you don't really have it."
I have seen packs go to the bottom of the ocean, fall of cliffs, and much more.
There are literally thousands of reasons or situations that can separate you from your pack. warm day you sit down on a log drop your pack, only to realize that there is a bee or wasp nest in the tree. (yes I have seen that one). You sit down keep your pack on you this time, and start to eat a snack, next thing you know a brown bear decides he wants that snack. dropping your heavy bag will be the first thing you do just to lighten your load.
getting separated from your backpack, car, ATV, or RV. is always a great possibility. I personally used to have concerns about loosing my truck to a river or lake, knowing that in my truck was where I kept all of my gear for survival or emergencies.
Often I get heckled for the gear I carry on me. But when you see a person have to ditch from a sea kayak and only able to keep with them what is on them,, you start to understand.