They are hitting off servers which are in charge of alot of things. A server host can own a specific battlegroup for example, but if the host gets hit off.. everyone in the specific battlegroup will witness lag. Or random DC's. Every host has it's own server. You can look at it like a piramide. There are all kinds of layers and sectors as it were. If they hit off the main host of Blizzard they can hit the entire system off. They aren't hitting Blizzard HQ, but a server connected to it which can cause alot of uncontrollable errors.