This was the result of a no-knock warrant: http://en.wikipedia.org/wiki/No-knock_warrant , which allows police to enter a home without prior warning, on the theory that giving notice will allow people inside to destroy evidence. They are almost uniquely a product of the drug war. One can imagine very few situations outside of drugs were evidence could be so easily destroyed as to justify a no-knock warrant.
(ie. You can't exactly flush a kilogram of cocaine and a triple-beam down the average toilet in a matter of minutes)