He didnt choke per say in many of those. That is the wrong word. In many of them he simply underperformed, or was outplayed.
RG 2012- Nadal on fire that season, one of his best clay seasons. Djokovic struggling after his grandfather's death. Realistically this one is a write off, was never winning this one.
Wimbledon 2012- Played a pretty mediocre to average match against Federer in the semis. I think he even had many more groundstroke unforced errors than Federer which is kind of comical. Federer in good form, not quite as good as the final but good enough. Pretty straightforward.
U.S Open 2012- Struggled with the conditions more than Murray it seemed. Was a shame as this was the first time since the winter hard court season he looked to really be back in form.
French 2013- Was outplayed. The match stats clearly show that. Still might have stolen the win if it wasnt for that overhead, but this is a shot he had trouble with until recently and finally seems to have fixed completely. He also blew an overhead on match point down against Nadal at the 08 Olympics and some other crucial times. Maybe this was the moment he decided to work on it in practice to master the shot since I dont recall any problems since.
Wimbledon 2013- Just a horrible performance. Not sure what to speak to this. Nothing against Murray who played well and could well have won anyway, but wow this was just bad. No way a choke though, just a really bad day for whatever reason.
U.S Open 2013- This is the first one he might have "choked" in the true sense. He got himself in a potential winning position late in the 3rd set and blew it.
Australian 2014- Tough match, could have gone either way like most of his matches with Stan in slams. Usually he wins out in these, today he didnt.
French 2014- Was sick which undoubtably affected his performance, too bad as Nadal was beatable here.
U.S Open 2014- Came up surprisingly flat against a very in form Nishikori who played maybe his best match ever to date.
Honestly U.S Open 2013 is really the only true "choke" with what happened at the end of the 3rd set. The others whether through his opponents play, his own, or whatever factor, he just wasnt the better player on court that day regardless.
Djokovic was never a Novotna or Sabatini like choker. What is more correct to say is from 2012-2014 he had trouble bringing his very best when it mattered most, outside of Australia. He also was not able to carry the full confidence and momentum of 2011 forward, and was having trouble living up the pressure he maybe put on himself to duplicate 2011. That is a totally different thing than choking though IMO.