I've come to the conclusion that he must have had a heart attack there at the end. However, I am worried that he may have been sick for the past few days, and such sickness could have brought about the heart attack. I don't know that much about chicken biology, so what usually causes a chicken to have a heart attack? With that, I am also starting to believe that he could have had a genetic issue. He was a mix between a RIR hen and a Polish roo that we hatched on our own, and another one of the birds we hatched has a cross beak, leading me to believe he could have had a genetic issue. Curious as to if I should be worried that he had something bring about the heart attack...