Just a culmination of events really.
Johnson bright, young talent with the world at his his feet.
Awarded with a reported 50,000k a week contract at such a young age, he had so much surplus cash he probably didn't know what to do.
An unfortunate niggly injury occurs. He's still a young lad, he has loads of free time as he can't train, he's loaded, he's young, he has young friends...hardly anything better than to go for a piss up with your mates at that age.
However these piss ups occur more and more regularly, puts on abit of weight but the piss ups don't stop.
A portion of the blame lies with the player, but alot of the blame must be put down at the feet of the club for not helping him.