Board Thread:Questions and Answers/@comment-31155663-20170810162058/@comment-31337598-20180921201213

IIRC, he died of old age.

It was first mentioned in the GO movie (which takes place after the quarter final against Genei Gakuen) by Endou before he enters with the rest.

It was mentioned again in Chrono Stone around episode 6 or so by Haruna.