Under pressure, people can abandon their inhibitions, cut corners, and loosen up their moral standards. In other words, they are more likely to engage in self-centered behaviors as opposed to pursuing the common good.
People adopt moral standards that dissuade them from unacceptable behaviors. Under normal circumstances, they think sensibly about the costs and benefits when making decisions. However, under pressure, people can be depleted of the cognitive resources they need to act ethically and resist temptations. See my article on the much-debated “muscle metaphor” of willpower.
When people are in that state of emotional and psychological anxiety, the brain goes into a defensive mode. With that, they are more likely to engage in self-interested behaviors that they would otherwise avoid, especially if the payoff for such behavior is high, and the odds of getting caught and punished are low.