I’m wondering why people are more likely to fail when they know people are watching and or counting on them. For example in a football game where there is 2 seconds left in the game, and team A has the ball and they’re 2 points behind team B. They go for a field goal and the kicker misses by a mile after going say 20 for 20 field goals all season before that kick. What explains the cause for underperformance when the stakes are high, or someone is watching, etc?
There are quite a few studies on this, but a nice recent one is this: Lee, Taraz G., et al. "Limiting motor skill knowledge via incidental training protects against choking under pressure." Psychonomic bulletin & review 26.1 (2019): 279-290.
They consider a few possible mechanisms, but end up leaning towards a 'high incentives are distracting' story. (Another candidate that can't be ruled out is 'high incentives encourage top-down control processes, and those are just not very good at driving motor sequences'). The main evidence hinges on differences between implicit and explicit learning of a key-sequence task: only the people who got explicit instruction show choking under pressure, even though both groups are doing the same task. If you just want the final take-home, it's "our results lead us to conclude that a training regime that seeks to restrict awareness of movement strategies and structure could lead to more stable performance under pressure, potentially via reduced performance deficits in the face of distraction." but underneath that claim there's a bunch of cool structure in the patterns of performance across the two groups that they dig into in some detail.
I think the answer is pretty easy: stress. As Wikipedia states it,
When people think the demands being placed on them exceed their ability to cope, they then perceive stress.
The player might feel that the expectations of them are too high and also if they fail, that the negative consequences will be big in terms of being blamed etc.
This stress then occupies certain part of player's mental capacity, which means they are not fully concentrated on the task. I'm sure there's plenty of literature on the effects of stress on performance.
Now, certain amount of stress can actually be beneficial for the performance because it increases arousal, but the Yerkes–Dodson law tells us that if there's too much arousal, performance will decrease.