I think it may be a psychological effect on how much stock people are putting on him to be the one to take out Jon Jones. The fact he was the first fighter whom Jon Jones wasn't openly vocal about firing back, like, for the first time JJ even showed signs that he was 'terrified' of Tom, with people making hundreds of "wow, what does Tom have that made even Jon turn into a scared fighter??" ... After the Aspinall vs Sergei fight, Jon Jones was a huge favourite vs Aspinall oddshwise, still. However, after Aspinall defeated Curtis, and with Jon's demeanor of never bragging and never saying openly he would beat Tom, hinting retirement instead, affected the betting system too, changing massively from a 80% JJ vs 20% Aspinall to a 52% Aspinall vs 48% JJ, more or less. JJ had said after he defeated Reyes, in a fight that he needed to push himself beyond his limits to win, like Gus 1, that he had gone through a lot of psychological reflection. And said that he'd try to show himself as the one no one can beat, and hide his fears in many press conferences, though with constant therapy and reading and all, Jones found out that showing he is insecure and fears losing like every fighter, works better for his mentality during the fight.
So idk, I think that when the expectation is also so big on you, like it is with Aspinall being the one to destroy JJ and all, the fear of failure also brings an insecurity. To be factual though, Aspinall said he has always been scared when fighting, like, in the Sergei vs Tom press, Tom said that he is scared, but that one of his strong sides is that he thrives in fear, that when he is within the biggest pressure and biggest fear moments, he shows his best self, even mentioning an animal that, when backed into a corner, unleashes a lethal poison (I forgot the animal's name tho). But he mentioned that while maintaining a very confident and cool composure. It feels different when he opens up about possibly being "exposed" and having to think on an alternate path... Which to me, already, isn't the thriving into fear, like, enjoying to be in such a position mentality like vs Sergei, but feeling a little unsettled by the possibility of it happening.