Assuming all other things are equal (ie existing limit orders on the book are not pulled out), a big MARKET SELL order will be absorbed first at the highest (ie best) bid, then the next bid down (next best) and the best bid will naturally fall. As the best bid falls, market makers will immediately offer lower ASK quotes (and if taken up on these, they buy at lower prices).
Quite apart from the above (ie movement driven by market orders), by definition market quotes (and best bid and ask) will move to any place that the aggregate of market makers choose to move them. Before economic news you can often see the best bid-ask widen, and the total number of limit orders lying on the book can drop significantly in the moments before a news release. So, there is less liquidity, and if the news is unexpected, (i) lower liquidity, (ii) lots of market orders and (iii) market makers' reaction (ie where they place their quotes) will all contribute to immediate volatility.
One theory says that market makers are typically agnostic about the market - they seek an equilibrium point where buying interest equals selling interest, and at equilibrium, they can continuously buy and sell, pocketing the bid-ask spread and keeping inventory low (ie as market makers they don't want to assume a position (directional risk) because their business model is to make the spread, not to take a directional view).
