He just left the band during the mini album’s recording and the other members couldn’t reach him so they had to quickly recruit a new vocalist. It seems like by that time he had arguments and clashes with most of the other members, especially Ao. He also seems prone to angry outbursts so I guess he’s not the most stable soul to work with…
(info taken from Aaron’s wonderful new website dedicated to Shuuji-1/other obscure acts)