You're both fixating on the specific example and missing the general idea: the neural pathways that let you not remember 100% of your drive home, forget your purse somewhere, go to the grocery store and forget the one thing you went for in the first place, and forgetting something is in your backseat are all the same part of your unconscious mind. If enough things go wrong at the same time, in the wrong ways, the part of your brain can take over tasks that aren't immediately urgent in that exact moment.

The article has a few segments that describe it well. This one came to mind:

There’s a dismayingly cartoonish expression for what happened to Lyn Balfour on March 30, 2007. British psychologist James Reason coined the term the “Swiss Cheese Model” in 1990 to explain through analogy why catastrophic failures can occur in organizations despite multiple layers of defense. Reason likens the layers to slices of Swiss cheese, piled upon each other, five or six deep. The holes represent small, potentially insignificant weaknesses. Things will totally collapse only rarely, he says, but when they do, it is by coincidence -- when all the holes happen to align so that there is a breach through the entire system.

On the day Balfour forgot Bryce in the car, she had been up much of the night, first babysitting for a friend who had to take her dog to an emergency vet clinic, then caring for Bryce, who was cranky with a cold. Because the baby was also tired, he uncharacteristically dozed in the car, so he made no noise. Because Balfour was planning to bring Bryce’s usual car seat to the fire station to be professionally installed, Bryce was positioned in a different car seat that day, not behind the passenger but behind the driver, and was thus not visible in the rear-view mirror. Because the family’s second car was on loan to a relative, Balfour drove her husband to work that day, meaning the diaper bag was in the back, not on the passenger seat, as usual, where she could see it. Because of a phone conversation with a young relative in trouble, and another with her boss about a crisis at work, Balfour spent most of the trip on her cell, stressed, solving other people’s problems. Because the babysitter had a new phone, it didn’t yet contain Balfour’s office phone number, only her cell number, meaning that when the sitter phoned to wonder why Balfour hadn’t dropped Bryce off that morning, it rang unheard in Balfour’s pocketbook.

The holes, all of them, aligned.