Why are the cookies flat again?

My husband makes chocolate chip cookies every Thursday night. He takes half the batch into his office on Friday so he doesn't end up eating them all (at least he doesn't eat them all at home. I have no idea if he then eats them all at the office). He's been making a batch of chocolate chip cookies every Thursday night for about 3 years now. There was a time when he also made them on Mondays. So he has had lots of practice. He's tweeked the flour weight, the softness of the butter, the amount of time spent creaming, the time & speed of the mixing, the timing of the baking, the right sized scoop. The past few months he has made excellent cookies - soft, yet crisp, with a decent rise & nice consistancy. But the last 3 batches have all been flat - spread out puddles of dough, with chips sticking up at random. Why? The flour is no more or less humid than it usually is and he weighs it. Its the same local organic butter we always use, same sugar, same brand of eggs. What could be making the cookies flatten out all of the sudden?