I have had things like that happen to me before. It usually results from me strapping items on my seat with bungee cords. I once lost a very heavy chain and lock when I hit a bump and its inertia overwhelmed the bungee cords and it flew off onto the freeway, never to be seen again. Another time I had some silk underwear strapped to my seat with a bungee net and after a hundred miles of riding it got pulled out of the net by wind buffeting. It then got caught up in my rear brake disc and caliper. My K100RS slowed down a little, so I just gave it more gas. When I finally arrived at a parking lot to check out the view of Crater Lake, I couldn't push my bike around. The silk had melted into the caliper and completely gummed up the rear brake. I was able to clean it out, but it was a real mess.
So now I use the RKA seat bag, although it does reduce your seat room a bit. It works really well and even when expanded to be about a foot high, I can carry a weeks worth of groceries, including canned goods and bottles of juice, without the bag moving around. However, I can not swing my boot over the bag when it is expanded, so I stand on my left foot, hold my right foot with my hands and kind of carry it over the rider's portion of the seat. Not very graceful, but it works and I haven't fallen over yet.