I'm shocked! But, I blame the record company entirely, they worked them absolutely too hard.
They're all young lads still, and they were riding a huge wave, they could easily have taken time out between releases - if the record company had let them, that is - instead of touring constantly and doing dancing and singing every night, and living in hotels and never getting to see their friends or family. That can't be easy, no matter how good friends you are with your bandmates.
I think if any of us had the prospect of travelling and working day in day out for years with just our work colleagues and rarely getting a day off, and when you do get a day off, your friends and family aren't near you to spend time with, that would be an exhausting situation to be in.