From the pics it looks like a bit of seasonal leaf drop but it also looks like its happening too fast because the nitrogen levels in the plant and soil are likely low. Feed it. The yellowing is probably mostly here to stay, but it may correct itself a little for a short time and then just go back to yellowing. Id guess the other ones arent far behind so id probably feed aggressively for a week or so and then back it off in hopes the others dont follow suite. Ive seen plants that yellow early finish just fine so dont worry too much about it, just feed more like supertramp says.