Mummytigger, it's explained in some of the articles, and is attributed to two things - a strange quirk of human memory, and a change in road safety law.
Basically when people are overtired and perhaps stressed because of work or unemployment, or difficult finances, or a difficult home situation such as ill relatives or whatever, they start to function on autopilot to an extent, and perform routine tasks almost mechanically, with one part of the brain, whilst another part of the brain that should be processing important information about the child's presence chunters away working on the problem that is bothering the individual, whether they want it to or not.
This means that their memory starts being unreliable - one psychologist describes this as the 'Swiss Cheese' theory - you can have a few holes ina Swiss Cheese and it is still fine, but add too many and basically the structure collapses. This could happy to any single one of us, which is why it is so worrying. They say if you can forget your keys, then you are technically capable of forgetting your baby in the back of the car given the right set of (admittedly extreme) circumstances.
Add to this a change in road safety law whereby cars have airbags and babies are required to be transported in the back, out of sight and out of mind, and you have a kind of perfect storm in which disasters like this can happen more readily. So the message is that if you really care about your children, try to avoid stress as much as possible, and be aware that we are all capable of such a terrible mistake and train yourself to look out for it in the same way that we look out for cars before we drive away from the kerb.
But please don't criticise these parents, all of whom are very like us here on MN, but additionally consumed with the most horrendous guilt. Show some compassion.