There is always a possibility the Koi dies due to lack of oxygen. Some customers believe a waterfall is enough for oxygen. In my opinion, it is not. The more aeration the better for your Koi especially during summer time. Your entire population might die overnight because of lack of oxygen. From our experience, massive die-off can be caused by either lack of oxygen or high levels of chlorine. This happens when you forget to shut off city water when you change water which has high levels of chlorine.
Read more in our pond oxygen guide.