"The Gnurrs Come from the Voodvork Out" is the title of a 1950 short story by Reginald Bretnor, part of his comic Papa Schimmelhorn series. Papa Schimmelhorn invented a secret weapon — a musical pipe. When it's played, gnurrs swarm out of the woodwork and eat the enemy's pants.
So the gnurrs became a fannish catchphrase. Sometimes they signify a particularly harried situation, especially one that feels like a lot of hungry little animals all pouring out to gnaw away in a demoralizing fashion. Sometimes it's just fun to say, "The Gnurrs Come from the Voodvork Out!"