Big Drop is the unique feature presented in Tony Hawk's Pro Skater 2. Due to players dislike, this feature was removed from the upcoming titles. However it made its return in Tony Hawk's Pro Skater HD as it was claimed to contain all the features from two initial games.
THPS series was originally presented as a simulator, hence the attention to skater stances and other specific things. It is known that height which allows skater to land the trick successfully is limited. Big Drop feature was most probably impressed by Jamie Thomas' Leap of Faith ollie footage.
When skater ollies from high ledges or launches himself high enough off the slope, there is a chance of Big Drop icon to appear, that will cause skater to bail if nothing was performed to avoid it. There are several way to avoid it: