I have just found this interesting information on Bulbapedia (credits goes to them) and it has something to do with the move Sky Drop.

Alright, so if your Pokemon knows the move Sky Drop (which are most likely Flying Pokemon, like Braviary for example) and used it, it will swipe the Pokemon away into the air. Now, if your Pokemon (or the opponent) uses the move Gravity, the Pokemon that used Sky Drop cannot finish the move and what's more, the Pokemon that was taken into the air by Sky Drop cannot take action at all.

This can only take place in Double or Triple Battles and therefore, Sky Drop is banned for the Random Wi-fi Match. For more details, please have a look at this video here.

This information also works for Pokemon Black.