Think of it this way, if an area of the lake due to circumstances (high algal bloom and subsequent die off) has used up all the available oxygen, fish do not have reasoning power to stop and think about what is happening and what should I do. Just like if you were walking around outside, and all the oxygen was removed from the air you are breathing, you don't know what has happened, or how far it extends, you only know you can't breathe, how long could you go without breathing. Its not an uncommon thing to happen, has happened before and will happen again.