From recollection there was an ask sum of around $16 million USD just prior to the collapse. Despite this the heavy volume of trading didn't kick in until it first looked like the dip was bottoming out so I'm going to hazard a guess that much of that ask depth (well as much as could be managed given MtGox technical limitations) was removed as the price was plummeting therefore it didn't need much panic or speculative selling to drive the price down pretty low. There was then an immense amount of buying at the first mini recovery until it became obvious for whatever reasons the bears weren't done yet so once again buyers stepped back and withdrew to watch and see how low it was going to go. With MtGox as (to date) the main price driver out of action there was no need to rush, no need to be afraid to miss the boat on buying back in and also it probably took some time to reassess how they judge the current market, many for the first time looking at prices over multiple exchanges prior to deciding how to play it.
What do you do if you want to sell more coins than the market can take at any one time? You put each dollar you got from selling a batch in as a lower bid to let things look solid
If you checked the Volume before the crash it was almost a straight line, linear rise of bid depth with price going lower. Almost to perfect a line for a human to draw.
We see similar things happening at bitcoin24 right now. People putting 100 coin bids in 5 bucks below the price, bids fill in above and once a few coins are sold at the now higher price the wall disappears. Put your tin foil hats on and be prepared, ugly action ahead.