Per the NYT:

Mr. McCain's aides disclosed the news at the same time as Hurricane Gustav struck land in what they said was an orchestrated attempt to minimize attention to it.

Mr. McCain's campaign, which has shown itself adept at handling the news media, tried to influence coverage of the disclosure by releasing it as Hurricane Gustav was slamming into the Gulf Coast. (The Palin news was not mentioned on the "CBS Evening News" until 15 minutes into the newscast). It was also by every appearance tucked into a series of problematic tidbits released about Ms. Palin's past, including news that her husband, Todd, was arrested for driving while impaired in 1986.