We have no idea what was in her head, but I have to wonder if there was some deep-seated resentment directed toward the child, perhaps the mother felt her career was being infringed upon; there could have been some hostility with her husband, who knows. I just think there’s more to it, whether she subconsciously didn’t want the child or she’s just a stupid and irresponsible person. We think about the things that we care about, that mean something to us. That she obviously didn’t remember she had the child in the car for that long a period of time says a whole lot.
Occam’s razor states “All things being equal, the simplest solution tends to be the right one,” or alternately, “we should not assert that for which we do not have some proof.” In other words, when multiple competing theories are equal in other respects, the principle recommends selecting the theory that introduces the fewest assumptions and postulates the fewest entities (although this is not always the same as simplicity). You’re overthinking this - it really could be as simple as she got distracted by work and believed she took the kid to the sitter.