Well it's not supposed to be random, anyone coming off a domestic flight is supposed to be checked for having an onwards connection, due to some obscure rules relating to customs and excise: I think the underlying rationale has now gone, but there was a requirement to ensure domestic passengers didn't wander off into duty free shops, and so HAL still checks people to see if they have a valid reason to be airside. It's not a very rigorous check, an itinerary print out is fine, but late at night, early in the morning there is often no one there to do the check.