I suppose gotogi layered stones might always have the possibility of doing this, but still surprising. Was the stone soaked at all, or was the environment especially dry or hot, or did the stone dry with sun on the top or something?
Can't see in the photo but is there a wooden base? Of course it's meant to stabilize and protect but in this case I'm wondering if it's what lead to the split, the attached side expanding and contracting differently than the top part? Just a guess, but personally I don't like my stones on bases if possible. (Easy enough to sharpen on a flat non-slip surface.) But your stone stone looks like it's a thin one so requires a base. If there's a next time, there are thicker Takashima available.
I'd glue it if I were you. Would still take you quite a while to get down to the glued middle part with normal wear, and when glued it should look fine again.