For example, in a tournament situation where me and the other player are down to the final match or final round. I tend to do not as well in those situations compared to a situation where the stakes aren't as high.
I basically do the same. In DOA, when it's down to the final round and both my opponent and I are down to our last bits of health, I tend to whiff holds, be predictable with my offense, and just overall choke. I guess, like Brute said, it's basically just nerves.
It's weird, though; in SMITE for example, I do not choke as much when it comes down to a 10 v 10 ticket difference in Arena. I always just go to the team portal and camp out until someone is forced to make a move. Once they do, I do in, still basically on defense, and throws out all my abilities at the most squishy person in range and hopes for a kill before heading back out before I make a mistake and gets killed.