I did something similar this evening. I've got the little saddle pack with a blinky light hanging off of it on my bike... For some reason, my saddle was sliding down like crazy, so every couple miles, I'd stop, undo that velcro strap, lift the seat up, tighten as hard as I could, tighten velcro strap... move on...
And then one time I don't know if I hadn't done the velcro strap up right, or if the seatpost just slid down that much - but I start hearing a sound and there's something wonky about the bike, so I brake and get off the road as quick as I can....
And the seatbag was dragging on the back wheel, which really means my blinky light had been dragging on the wheel, so the blinky light had black plastic fused to it and gouges going down it. One of the gouges went through all the way.
Definitely not as expensive as your shorts though.