Two months is certainly long enough for the mind and body to develop a habit, but it's also a short enough time for the muscle memory from before that time to still be in place. So if they went about their day following what used to be their schedule and patterns from before this baby was adopted, I can see that happening.
I have a theory about these cases, at least the ones where the child was honestly forgotten about. My theory applies to the overall increase in these cases, not about any specific case. I'm thinking about how life has become so much more fast-paced, people try to normalize a certain amount of stress and to keep all their "balls in the air" by sheer willpower rather than by actually getting enough sleep, enough exercise, and having a life that doesn't exceed their mental and scheduling means.
At the same time I won't be surprised if it turns out there is more to this story. Even if my "stressful life" theory is correct, that doesn't provide an explanation for individual cases.
So sad.
MOO