Yikes, Ewan! What carnage, and in just a moment's time!
Your tale is instructive to all of us, and a reminder that even seemingly innocuous items can be the catalyst for Much Larger Things if it all comes together wrong. So sorry for the bad experience, and hope no one was injured (apart from the wheel and the panniers).
My Dutch touring partner had front rollers (has since added them to the rear as well, replacing an older set of bags by another maker) and was spooked by the possibilities of the straps hanging in their keepers, so he removed the straps and simply clicked the Fastex buckles to each other at the top, after rolling. That little hook was still there on the face of each bag, though, and I sometimes wondered if...! Now I know. No matter how unlikely or infrequent, Things can Happen.
I went with the cap-top SportPacker and BikePacker series, but still take great care to "safety wire" all items I carry outside my bags or under the lid: I use a little mini-carabiner to secure any loose drawstrings (say on my chair stuff sack, tent carry bag, etc.) so a) the drawstrings won't feed into the spokes and get caught with horrible outcomes and b) so I won't lose these items if they should work loose from their regular straps. Also, thanks to Ortlieb's most recent warning, I have stowed my HB bag strap inside the bag, rather than wrapping it 'round the bottom.
Best,
Dan.