There was a spate of moped-mounted Yoofs robbing panniers off moving bikes down the Caledonian Road a Several of years back. Used to secure mine with a toe strap when I had cause to go that way.
I'm in the habit of crossing the straps of roll-closure Ortliebs with the one on the opposite side of the rack for that reason. Also useful for strapping things down on the top of the rack, and preventing pannier ejection during Comedy Off-Roading (with Mildly Inappropriate Luggage) incidents.
My daily driver panniers are Alturas, with their slightly faffier push-button release.
