It's likely a playful twist on the phrase "open up a keg of beer," which means to start a party or celebration by opening a large container of beer.
The humor comes from the incongruity of opening a keg of nails, as nails are not typically associated with parties or celebrations.
If someone uses the phrase "open up a keg of nails," they are probably being sarcastic or humorous, implying a situation that is chaotic, messy, or difficult.