He was the only one having to give face-to-face noms... The one thing we know all the housemates hate doing... And he decided to make a big humble, caring-uncle thing about it. Which was a terrible idea... I mean, I could see what he was thinking but if he'd have just said, "My first nomination is X, they fart in their sleep and it keeps me awake." then he'd have gotten a few laughs, whoever he'd nominated would have been a bit pissed off but would have got over it quickly in a "well we all have to do it and he obviously couldn't think of someone to nominate" way and he'd have been golden.
His biggest misstep though was his reasons for nominating Darren... Something I still don't think he comprehends and something Darren actually hasn't even explained to him that well, focusing instead on the whole "I thought we were mates" aspect of it...
...ADDICTION. Darren is a recovering alcoholic and substance abuser. This is a huge thing in his life and doubtless a huge thing for all his friends and family... And John's nomination was centred around "seeing hIm starting to slip back". This of course is a massive thing to say about someone in Darren's position... The worst thing to say really... Which is why Darren immediately started looking up towards the cameras and asking big brother to make sure they made it clear John was actually talking about cigarettes, knowing that his friends/family on the outside who are doubtless always worried about a back-step didn't get the wrong idea. The fear of 'slipping back' will be on their mind constantly same as its on Darren's and John just casually announced it was happening. When arguably it wasn't.
It was a silly and very cruel and thoughtless reason to nominate. I don't actually think John knew that... I'm not even sure he does now...