From Glitch City Laboratories
The term "BG event." refers to one of several error traps used to prevent any glitchy effects of an error or to prevent the game from freezing.
This phrase has been found within the coding of Pokémon Gold, Pokémon Silver and Pokémon Crystal. However, what causes this error message to appear is currently unknown. Its name suggests itself as a placeholder for when the player speaks to an object or NPC whom has no defined text and is meant to change the details of the map background. For example, Lance when he reveals the secret entrance to the Rocket base in Mahogany Town, however this is unconfirmed.