something i noticed while waiting for the news from japan in BTC trollbox this morning - a big dump happened about 30-60 minutes before the "news" was even announced.
The first large reactionary price drop occurred on various markets approximately when gox's office relocation was first announced. This was not seen as widely or as quickly as the later "no withdrawals yet" announcement because it was placed only on gox's support page (not the home page).
The 'big dump' you are referring to may have been the first drop I noticed, or more likely a later reaction in between. (IIRC, the two announcements were spaced much further apart than the 30-60 minute gap you mentioned.)