I recently found out about Caruana's performance in the Sinquefield Cup - 2014. I only started watching chess recently. I was convinced that Carlsen is head and shoulders over everyone else when I started watching two months back. Now I've realized anyone above 2750 is very dangerous. Recently I came to know about Caruana's performance in 2014 and I am wondering why he was not able to maintain that level of performance .
TL;DR: Caruana says that his performance depends on 'luck', which can be boiled down to Caruana's effort in the game compared to his opponent's effort. And by effort at this level, he mostly means fighting spirit and opening preparation, but also external factors affecting the players' psychology.
This interview records Caruana's changing fortunes in his own words:
Interviewer: You’ve managed to be the 3rd highest rated playe[r] in chess history at your young age. To what do you owe this success?
Caruana: I think I was having an extremely good period. I won 40 points in one tournament. It could have gone another way - I could have just had a few good tournaments, but all my success was basically concentrated on one tournament and that’s why I (laughs a little) won seven games in a row and my rating went up to 2840… I still wonder why I had that result and some results after it were very good, like the European Club Cup and after the Grand Prix. For some time I was playing extremely strong chess, probably stronger than I deserved (laughs). Maybe I had some luck as well! Because I had the strong period, and for a couple months after that I was struggling. In Wijk aan Zee, Zurich the next year I had like a six-month period where I was struggling to show good results, but for that especially one tournament (Sinquefield Cup 2014) I was unstoppable.
Source: chess24.com, emphasis is mine.
So Caruana says that he put a lot of effort into one tournament, the Sinquefield Cup 2014, but after that it is hard to maintain such a dominant position in chess. From the same interview, Caruana says this about fluctuating performance:
Let’s say you’re playing an opponent who is 2750. There’s a chance that one day he will play like, let’s say, not 2750 but 2300, 2400 or 2500 – he’ll just have a very bad day. Just a recent example - Nakamura against Ding Liren in the last round of the Sinquefield Cup was just an example of a very strong player having a very bad day, but I think it all balances up. You might have cases where your opponent is playing very poorly, but also cases where your opponent plays just unstoppably and doesn’t give you a chance, or you fall into preparation and lose because your opponent just had a new opening idea. Or the reverse! You win because of your new opening idea. I think in general it balances up, but there are ways to increase your luck just by fighting and never giving up. The more resistance you put up the more chances there are that your opponent will screw up.