Because they received transmissions from Earth containing episodes of "The Nanny". Fran Drescher's voice in the form of electromagnetic waves caused several of their systems' suns to go very supernova and some collapsed into black holes. Seeing as they used solarformation and Dyson spheres to regulate their needs, blowing those up was considered a declaration of war. The only reason they've never pushed all the way to New York is that they consider Niles' sarcastic retorts witty and funny.