I have Ortliebs with a buckle for a secondary retention strap. I use it during rough road (gravel-fire road) tours) and usually not on road tours. I had a (inexpensive) pannier jump the rack in the 80's on a washboard gravel descent, spilling the contents down a steep gully. I lost some gear that I couldn't recover. I doubt orts could jump the rack, but the secondary strap limits the rattling on rough, high speed descents.
lesson learned.