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.
How is this a better idea than simply halting trading?
My thoughts exactly.
It's not like the hackers could magic out the fiat without them sending it out to a bank. Just mark account transfers for manual approval over a certain limit.