Now in some cases, parents have brought their little kids to the game. The kids are bored (because they're kids and it's a baseball game) so the parents spend the entire time attempting to "sell" the game to the kid by explaining every moment, saying "look, let's watch and see if he can get him out" or telling the kid what to cheer. While this can get a little old...it makes sense. The kid doesn't know what he doesn't know, and probably wouldn't think to even ask. While these "over-explaining" parents can sometimes be a bit much, I see where they're coming from.
The "experts", however, are another story. They're the ones that have found a friend, or a seat-mate, who knows just slightly less about the game than they do...and they've decided to use it as an excuse to share everything they know about the game. Whether its explaining the history of the Pirates franchise, making arguments they pulled off of Sports Talk radio without referencing their source, chuckling when someone asks a legitimate question, or just literally narrating the game without any sort of prompting or provocation.
I can't get too annoyed at the "experts" because I'm well aware that I slip into this mode sometimes, especially when I'm at the game with someone who rarely attends. But even so, I like to think I'm self aware and use a few disclaimers ("if I'm over-explaining, let me know" or "stop me if you already know this"). I think its just an interesting phenomenon that people...like to explain things. They like to learn something, then immediately go find someone who doesn't know that thing, and "teach" it to them. I can't tell if altruistic, or boastful. Perhaps it's lazy, or noble. But for whatever reason, whether its a game, the cafeteria, or just everyday conversations, there's always a plethora of "experts" ready to share their knowledge with the world. Some people enjoy listening...others spend the whole time thinking "yeah...everybody knows that".