It was already explained in another thread, it's a security measure (of rather dubious quality) they put in for when price is falling too fast. This is the kind of "solution" that makes software engineers around the world scream out in agony.
I sat there trying to cancel re-appearing orders for an hour... thankfully it did seem to stop finally, and I've been able to withdraw my bitcoins...
You withdraw your bitcoins and so do a bunch of other people who are now spooked with Gox. Those who panic sold will later panic buy into a thinner ask wall, realizing that withdrawing via bitcoin is much faster and easier than USD. This will drive the price UP.
Anti-fragility at work.