Not enough photo detail of the rear wheel setup. With an older frame with minimal or no redundant safeties ("lawyer lips"), the weaker external cam QRs might not hold securely enough.
A few possibilities, based on stuff I've done myself through neglected maintenance and safety checks (although none has resulted in any problems)...
When inserting the rear wheel, it doesn't seat properly in the dropouts for various reasons. Maybe a bit of debris on the dropout or axle. And when I lock the QR it's not really secured. I discovered the booboo soon after starting a ride when I noticed the rear wheel had shifted askew, signaled by brake rubbing, or visible when I glanced down and noticed the tire nearly rubbing one chainstay. Once I found a bit of what resembled broken plastic or something jammed between the inside of the dropout and axle. Probably stuck to the wheel when I set it down in the grass during a flat repair.
A few days ago I noticed a seldom-ridden errand bike's rear wheel had a loose nut on the drive side. That's an oddball bike, an early-mid 2000s Globe Carmel. Globe was the bargain lineup from Specialized that had a brief surge of popularity during the 2000s hipster era. Most of the Globe lineup was functional city cruisers or single speeds. The Carmel appeared to have been intended for an early ebike, with a massively heavy aluminum frame, long wheelbase like a cargo bike (the Carmel resembles one of the Yuba cargo bikes, minus the permanently attached rack), and ridiculously wide rear dropouts (nearly 150mm wide). Apparently Globe intended the Carmel to be adapted to their plans for ebikes but the whole Globe project was phased out before that happened (per interviews with the former engineer assigned to that project).
So instead of a heavy duty, wide wheel, the Globe was fitted with a standard 126mm wheel on an inexpensive Alex Z1000 single wall rim, freewheel, longer axle, and a kludge of extension nuts to fill the space. That resulted in a rear wheel too flimsy for the intended purpose and it began popping spokes after about a year of riding.
I replaced it with a heavy duty affordable Wheelmaster wheel built on a Weinmann Zac-19 double wall rim, heavy duty Weinmann hub, massive spokes, etc. But it still needed that ridiculously long axle and filler nuts.
I noticed the other day the filler nuts were loose. Chances are it wouldn't have caused problems immediately. The bike has hefty safety lugs on front and rear dropouts (again, presumably the frame was intended for an ebike), so even the cheap exposed cam QR lever was still adequate as long as the skewer didn't snap or the retaining nut didn't strip out and pop off.
I'm planning to sell or give away that errand bike, so I'm going to switch to a proper QR, and use a bit of moderate strength Loctite on the filler nuts.
Anyway, now I try to be a bit more diligent about pre-ride safety checks. And if I forget and then remind myself after a ride has started, I'll pull over, undo the QRs on both wheels, set the frame firmly into the wheels, relock, check, etc.