This can occur due to a number of factors -
1. You do not want to be embarrassed by not being able to hit the weight you loaded up on the bar in front of a stranger.
2. You do not trust that you will get a safe spot from this stranger so as part of self-preservation the adrenaline kicks in.
3. You fear that the stranger will assist when you don't need it, so you enable beast mode.
(A combination of several of these scenarios are common occurrences)
"Whoa, you didn't come close to that last week...what gives?" queries Jefferson.
"Must have been the random spot adrenaline kicking in" replied Jimmy