I think it was the fact that Daniel hated the spotlight and wanted to work more in the background, leaving Darren to do all the publicity (which I know Darren resented). Also, I think they wanted to go in different directions, both personally and musically - Darren loves his songwriting and Daniel loves producing.

ALSO, just the whole overwhelming pressure of international fame. It's a hard thing to perpetuate for long periods, ask anyone...